The factors MBNL1 and U2AF65 bind alternative RNA structures to regulate splicing

M. Bryan Warf, Julien V. Diegel, Peter H. von Hippel, and J. Andrew Berglund1

Institute of Molecular Biology and Department of Chemistry, University of Oregon, Eugene, OR 97403

Edited by Michael Rosbash, Brandeis University, Waltham, MA, and approved April 14, 2009 (received for review January 14, 2009) Myotonic dystrophy type 1 (DM1) is a genetic disorder linked to a snRNP, the spliceosome selects another 3Ј splice site, or the CTG)n repeat expansion in the 3؅ untranslated region of the DMPK intron is retained if another 3Ј splice site cannot be defined) . Upon transcription in the nucleus, the CUG repeats form a (13, 14). stable RNA stem-loop that sequesters the RNA-binding protein We hypothesized that MBNL1 may act through a similar MBNL1 from its normal function in the cell. MBNL1 regulates the mechanism to compete with U2AF65. U2AF65 is a potential alternative splicing of many pre-mRNAs, and upon MBNL1’s competitive target of MBNL1 because a putative U2AF65 sequestration, the alternative splicing of many is mis- binding site appears to be in the loop portion of the stem-loop regulated, leading to disease symptoms. MBNL1 is known to bind which MBNL1 binds (5). We found that MBNL1 does compete directly to at least 3 of the pre-mRNAs that it regulates, but how with U2AF65 for binding of a region within intron 4. This MBNL1 binding mechanistically regulates alternative splicing is competition with U2AF65 is functionally important, because unclear. Here, we demonstrate that MBNL1 controls the splicing of recruitment of the U2 snRNP is reduced by MBNL1. Further- exon 5 in the cardiac troponin T (cTNT) pre-mRNA by competing more, we found that MBNL1 and U2AF65 compete by binding directly with the essential splicing factor U2AF65 for binding at mutually exclusive RNA structures. the 3؅ end of intron 4. When U2AF65 is prevented from binding to the pre-mRNA, the U2 snRNP can no longer be recruited and the Results following exon is skipped. Furthermore, MBNL1 and U2AF65 ap- U2AF65 Recognizes a Canonical Sequence Within Intron 4 of the cTNT pear to compete by binding to mutually exclusive RNA structures. pre-mRNA. We have shown that MBNL1 binds a stem-loop When bound by splicing factors, the 3؅ end of intron 4 can form located at the 3Ј end of intron 4 (Fig. 1A and B) (5). A putative either a stem-loop or a single-stranded structure. MBNL1 binds a polypyrimidine tract (py-tract) for intron 4, the likely U2AF65 portion of the intron as a stem-loop, whereas U2AF65 binds the binding site, exists primarily within the loop portion of this same region in a single-strand structure. Mutations that strengthen stem-loop (Fig. 1 A and B). This loop contains an uninterrupted the stem-loop decrease U2AF65 binding affinity and also repress sequence of 5 uracil and 7 cytosine residues and is 25 residues exon 5 inclusion, independently of MBNL1. Thus, U2AF65 binding upstream of the 3Ј splice site and 9 residues downstream of a can be blocked either by MBNL1 binding or by the stabilization of consensus branch-point sequence. RNA secondary structure. The affinities of U2AF65 for regions of this intron were determined by using an electrophoretic mobility shift (gel muscleblind-like 1 ͉ myotonic dystrophy ͉ alternative splicing shift) assay. U2AF65 bound to a 50-nucleotide sequence (cTNT 50mer) containing the putative py-tract with an affinity Ϯ lternative splicing is a fundamentally important process that (Kd)of310 30 nM (Fig. 1C, see supporting information (SI) many organisms use to increase proteomic diversity. Many Fig. S1 for representative gel shifts). Truncated versions of this A RNA that retained the putative py-tract, but removed another diseases are known to arise from the mis-regulation of alterna- Ј tive splicing (1). Myotonic dystrophy (DM) is an example of a run of pyrimidines nearer to the 3 splice site, showed an disease where the alternative splicing of many pre-mRNAs is unchanged binding affinity for U2AF65 (cTNT 32mer, Fig. 1 B C mis-regulated. The mis-splicing seen in DM is thought to be and ). Mutations that replaced the putative py-tract nearly B C caused, at least in part, by the mis-localization of the splicing abolished U2AF65 binding (UUCG Loop, Fig. 1 and ). Based on its location, the sequence of the region, and the factor MBNL1 (muscleblind-like 1) (2–4). MBNL1 is known to binding data and the truncations, it is likely that this region is interact directly with 3 pre-mRNAs that are mis-regulated in indeed the py-tract and comprises the binding site of U2AF65 DM, the cardiac troponin T (cTNT) (5, 6), fast troponin T (7), within this intron. and SERCA1 (8). It is unclear how many more of the mis- regulated pre-mRNAs observed in DM may also be directly MBNL1 and U2AF65 Bind Competitively to the 3؅ End of Intron 4. To bound and regulated by MBNL1. determine whether MBNL1 competes with U2AF65 for bind- We have recently shown that MBNL1 binds a stem-loop within ing at this intronic site, a UV cross-linking assay was used. intron 4 of the cardiac troponin T (cTNT) pre-mRNA. This While holding U2AF65 concentration constant, increasing the stem-loop is located directly upstream of exon 5, which is concentration of MBNL1 strongly reduced U2AF65 cross- mis-regulated in this disease (5, 6). The mechanism used by linking (Fig. 2). If the 2 could bind to the RNA at the MBNL1 to repress exon 5 remains undetermined. One mecha- same time, it was expected that a ternary complex of higher nism might entail a direct binding competition between MBNL1 molecular weight would be observed. However, because and other splicing factors. One of the best articulated models of the regulation of alternative splicing is sex determination in Drosophila melanogaster, where the protein Sex Lethal (Sxl) Author contributions: M.B.W. and J.A.B. designed research; M.B.W. and J.V.D. performed competes with the splicing factor U2AF65 at the 3Ј end of certain research; M.B.W., P.H.v.H., and J.A.B. analyzed data; and M.B.W., P.H.v.H., and J.A.B. wrote introns [reviewed in ref. 9]. U2AF65 is thought to be 1 of the first the paper. splicing factors to bind an intron and thus, helps define the 3Ј end The authors declare no conflict of interest. of the intron. In concert with other proteins that bind near the This article is a PNAS Direct Submission. 3Ј end, U2AF65 is responsible for helping to recruit the U2 1To whom correspondence should be addressed. E-mail: [email protected]. snRNP to the branch-point sequence (10–12). When Sxl inhibits This article contains supporting information online at www.pnas.org/cgi/content/full/ U2AF65 binding and the subsequent recruitment of the U2 0900342106/DCSupplemental.

www.pnas.org͞cgi͞doi͞10.1073͞pnas.0900342106 PNAS Early Edition ͉ 1of6 Downloaded by guest on October 3, 2021 A Intron 4 on the RNA substrate. A time course shows that this cTNT Exon 4 / / Exon 5 pre-mRNA forms A complex well (Fig. S2B). The ADML pre-mRNA substrate served as a positive control because it cTNT 50mer construct forms A complex robustly (Fig. S2B). In the absence of ATP, no CCC AGA CUA ACC UGU CUC GCU UUU CCC CUC CGC UGC GGC CAC UCC CUG AAC CUC AG -56 -7 -1 complex band is formed on the cTNT-Wt pre-mRNA, indicating putative branch-point putative poly-pyrimidine tract that this is A complex (Fig. S2C). At later time points, larger 3’ splice site bases in stem complexes (B and C) are seen on the ADML pre-mRNA, but the

UUCG Loop (UL) 1 cTNT 50mer cTNT pre-mRNAs are not robust enough to form these com- BCC CC C U U U C cTNT 32mer plexes in vitro before RNA degradation. U C U G 0.8 UL U C U - G The 10-min time point was chosen to assay the effect of U G C C - 0.6 C C G - U MBNL1 on A complex formation because RNA degradation was cTNT 50mer G - U C - G C G minimal and complex formation was optimal on the cTNT / 32mer - U C 0.4 U C C - G pre-mRNA (Fig. S2B). Complex formation on the cTNT-Wt C - G U - G U G - * truncations G - C 0.2 G C pre-mRNA was reduced by nearly 2-fold with increasing con- - for 32mer U C Bound Fraction RNA U C C A centrations of MBNL1, whereas little effect was seen on the C A C C 0 5’ * C C * 5’ 0.01 0.1 1 10 ADML pre-mRNA or on the mutant cTNT-4G pre-mRNA (Fig. C C C A G A C U A A U C C C U G A A U2AF65 (µM) 3A and B). This indicates that MBNL1 reduces the formation of Fig. 1. U2AF65 binds the putative polypyrimidine tract in the cTNT intron 4. A complex only on its specific pre-mRNA target, and does not (A) Schematic of the 3Ј end of intron 4 of the cTNT pre-mRNA. The cTNT 50mer perturb A complex formation in general. used for binding studies is indicated, and the bases in the stem-loop structure. The 3Ј splice site, putative polypyrimidine tract and putative branch-point are Stabilization of the Stem-Loop Reduces U2AF65 Binding Affinity and also indicated. (B) Structures of tested . (C) Binding curves of tested RNAs. Represses Exon 5 Inclusion in Vivo. We previously observed that mutations that increased the number of base-pairs in the stem- loop repressed exon 5 inclusion, independently of MBNL1 (5). U2AF65 cross-linking was significantly decreased and little or This led us to hypothesize that these mutations might repress the no apparent ternary complex was observed, it appears that inclusion of exon 5 by reducing the affinity of U2AF65 for the MBNL1 and U2AF65 bind competitively to the cTNT py-tract, in a manner similar to the repression mechanism used 50mer-Wt sequence. by MBNL1. To investigate this possibility, we tested the affinity A mutated RNA (cTNT 50mer-4G, Fig. 2) was used to of U2AF65 for the cTNT intron 4 py-tract using stem-loops with determine whether nonspecific binding of MBNL1 affected increased stability. U2AF65 binding. In this RNA, 4 guanosine residues in the stem A correlation was seen between the stability of the stem-loop were mutated that are required for MBNL1 binding (5) and and the binding affinity of U2AF65, with stronger stems nearly cross-linking (6), whereas U2AF65 binding and cross-linking abolishing U2AF65 binding (Fig. 4 A and B). The Stem remain intact. As expected, cross-linking of this RNA to Strengthen (SS) mutation stabilized the stem by adding one base U2AF65 was not affected by MBNL1, showing that MBNL1 pair to each end of the stem, increasing the Tm of the stem-loop specifically competes with U2AF65 only for the wild-type RNA by 5 °C, and reduced U2AF65 binding nearly 10-fold (Fig. 4 A sequence. and B). The No Mismatch (NM) mutation increased the stability of the stem by 10 °C and reduced U2AF65 binding to the point MBNL1 Inhibits ‘‘A Complex’’ Formation on Intron 4. The role of where a Kd value could not be determined (Fig. 4 A and B). It U2AF65 is to define the 3Ј end of an intron and help recruit is interesting that the mutation that more strongly stabilized the the U2 snRNP to the branch-point (10–12), forming ‘‘A stem structure had a larger effect on binding. This result suggests complex’’ (15, 16). If U2AF65 does not bind, recruitment of that U2AF65 binds the py-tract in a single-stranded region of this the U2 snRNP to the branch-point is usually compromised (11, RNA. As a negative control, a mutation (UUCG loop, UL) was 12). We therefore predicted that the competition of MBNL1 made that removed the py-tract from the loop to fully abolish with U2AF65 would inhibit the formation of the A complex on U2AF65 binding (Fig. 4 A and B). this intron. Furthermore, we found a strong correlation between An RNA construct was made that contained human cTNT U2AF65 binding affinity and exon 5 inclusion in vivo (Fig. 4 exon 5, together with 100 nts of the 3Ј end of the RNA upstream C and D). The NM, SS, UL mutations all showed minimal exon of intron 4. Sequences from a constitutively spliced intron from 5 inclusion using an in vivo splicing assay. This repression was chicken troponin I (TNI) were placed upstream of that sequence independent of MBNL1, because coexpression of expanded (see Fig. S2A for diagram). Radiolabeled RNA was incubated CUG repeats (which would sequester MBNL1) had no affect with HeLa nuclear extract, allowing splicing complexes to form on exon 5 inclusion for these mutations (Fig. 4D). These results indicate that inclusion of exon 5 can be regulated either by competition of MBNL1 with U2AF65 at the 3Ј end of the intron, or by the formation of RNA secondary structures that U2AF65 (µM) - - 2 2222 - - 2 2222 reduce the affinity of U2AF65 for its binding site. MBNL1 (µM) - 10 - 310.3 10 - 10 - 310.3 10 U2AF65 and MBNL1 Bind Distinct RNA Structures. To determine the U2AF65 + RNA effect of MBNL1 or U2AF65 binding on the stem-loop structure, a fluorescence-based assay was used. The fluorescent adenosine MBNL1 + RNA analogue 2-amino purine (2-AP) was used to replace a guanosine residue that forms a ‘‘wobble base-pair’’ with a uracil residue near the base of the stem (Fig. 5A). 2-AP was chosen as a 1234567 891011121314 spectroscopic probe for this purpose because its fluorescence is cTNT 50mer-Wt cTNT 50mer-4G strongly quenched when it is in a base-paired structure (here with Fig. 2. MBNL1 and U2AF65 competitively bind intron 4 of the cTNT pre- uracil), whereas its fluorescence strongly increases when it is in mRNA. MBNL1 and U2AF65 were UV cross-linked to the cTNT 50mer-Wt RNA a single-stranded environment. The guanosine residue near the and run on a denaturing gel. base of the stem was selected for replacement as no adenosine

2of6 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0900342106 Warf et al. Downloaded by guest on October 3, 2021 A B 2.0 MBNL1 (µM) 0 0 0.6 1.2 2.5 5 000.6 1.2 2.5 5 00 1.2 2.5 5 Time (mins) 010 010 0 10 1.8

1.6 A Complex 1.4

1.2 H Complex

[ Reduction of A complex Fold 1.0 1234 56 789101112 13 14 15 16 17 01.22.5 5 ADML cTNT-Wt cTNT-4G MBNL1 (µM)

Fig. 3. MBNL1 reduces ‘‘A complex’’ formation specifically on intron 4 of the cTNT pre-mRNA. (A) A complex formation on the cTNT-Wt pre-mRNA, ADML and cTNT-4G pre-mRNAs. (B) Quantitation showing the effect of MBNL1 on A complex formation on cTNT-Wt, cTNT-4G and ADML pre-mRNAs.

residues were available in the stem and replacement of this 2 zinc fingers from MBNL1 with a six nucleotide RNA suggests position had no effect on MBNL1 or U2AF65 binding or the that at least 2 of MBNL1’s zinc fingers bind RNA sequences stability of the stem. in a single-stranded structure (19). This crystal structure was Titration of U2AF65 to the RNA increased the fluorescence only obtained with a minimal RNA sequence that was not long of the probe nearly 2-fold (Fig. 5B). In general, the fluorescence enough to form a double-stranded structure, so it is unclear of a 2-AP probe has been shown to increase 2- to 3-fold when whether a longer RNA sequence would be in a different 2-AP switches from a base-paired to a single-stranded environ- structure. It might be possible that MBNL1 does bind the ment (17). Therefore, this result suggests that U2AF65 binds this cTNT stem as single-stranded RNA, but the protein quenches RNA in a single-stranded structure. U2AF65 was titrated to a the 2-AP probe so that the fluorescence does not increase. point where the RNA should be approximately 85% bound, However, this is unlikely, because 2-AP probes have not been

based on the Kd determined from the gel shift assay. A U2AF65 seen to be strongly quenched by direct protein binding when BIOCHEMISTRY protein that contains 4 mutations which inhibit RNA binding compared with base stacking and base-pairing in a stem (U2AF65–Quad Mutant or QM (18)) had no effect on the (17, 20). fluorescence of the probe (Fig. 5B). The recent crystal structure argues that it is probable that In contrast to U2AF65, titration of MBNL1 (to a concen- MBNL1 opens at least 1 or 2 bases in the stem. However, this tration at which 85% of the RNA should be bound) had no fluorescence data suggests that other portions of the stem significant effect on the fluorescence of the 2-AP probe (Fig. remain structured. We therefore conclude that at least a portion 5C). This result indicates that the stem structure remains intact of the stem remains intact upon MBNL1 binding, whereas upon MBNL1 binding. However, a recent crystal structure of U2AF65 binds to a completely single-stranded structure.

C C C C C C C C C C C C U C 1 ABU U U U U U Wt 32mer U G U C U C U C SS U C U G U C U G - U G U G U G - 0.8 NM C - C - - C C C C G C UL G U - G - U G U G U C G - - C - G C G C G 0.6 U - C - - U C U C G C C G - C - G C G C G U - G - - U - G 0.4 U G U G G - C - - G - C - G - C G - C U C U C G C U C - C A 0.2 C A C A C A 5’ C C Bound Fraction RNA 5’ C C 5’ C C 5’ C C Wildtype (Wt) Stem Strengthen (SS) No Mismatches (NM) UUCG Loop (UL) 0 T (°C) m 52 57 62 61 0.01 0.1 1 10 U2AF65 (µM) KD(µM) 0.31 ± 0.03 4.1 ± 0.9 >10 >10

C D100

80 Wt SS NM UL

60 +RT -RT +RT -RT +RT -RT +RT -RT + Exon 5 40 - Exon 5 20 % Exon 5 Inclusion 66 ± 1 13 ± 2 13 ± 1 4 ± 3 Percent Exon 5 Inclusion 0 Wt+ SS+ NM+ UL+ Wt SS NM UL CUG CUG CUG CUG 960 960 960 960

Fig. 4. Mutations that stabilize the stem-loop structure reduce U2AF65 binding affinity and repress exon 5 inclusion in vivo. (A) Schematic of different mutations, the thermal stability of each mutation and the binding affinity of U2AF65 for each RNA. (B) Binding curve of tested mutations. (C) Representative RT-PCR data from in vivo splicing of cTNT mutant series. (D) Quantitation of RT-PCR data, and data from CUG960 cotransfection experiments.

Warf et al. PNAS Early Edition ͉ 3of6 Downloaded by guest on October 3, 2021 2.2 2.2 A B U2AF65-Wt C C C C C C C C C U2AF65-QM U U U U 2 2 U C U C U C U C U - G U - G 1.8 1.8 C C C C G - U G - U 1.6 1.6 C - G C - G U C U C 1.4 1.4 C - G C - G 1.2 U - G U - 2AP 1.2 Relative Fluorescence G - C G - C Relative Fluorescence U C U C 1 C A C A 1 C C C C 5’ 5’ 0.8 Wildtype (Wt) 2-Amino 0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 0 100 200 300 400 500 Purine (2-AP) U2AF65 (µM) MBNL1 (nM)

Fig. 5. U2AF65 destabilizes the stem-loop upon binding, whereas MBNL1 binds the stem-loop structure. (A) Schematic of fluorescent RNA used to probe cTNT structure. (B) Effects of wild-type or Quad Mutant (QM) U2AFA65 on cTNT fluorescent probe. (C) Effect of MBNL1 on the fluorescent cTNT probe.

Discussion of single-stranded uracil residues (21). When competitively MBNL1 Regulates the cTNT Exon 5 Through Competition with U2AF65. binding with U2AF65, both Sxl and PTB are considered to The competition of MBNL1 with U2AF65 represents an exam- occlude the U2AF65 binding site, and sterically inhibit ple of an alternative splicing factor that regulates splicing U2AF65 from accessing its binding site. However, MBNL1 has primarily through modulation of an RNA structural element and a different binding specificity from U2AF65, and binds adja- not through direct occlusion of another splicing factor’s binding cent sequences primarily outside of the py-tract. We cannot rule out steric contributions to the competition between site. The competition of MBNL1 with U2AF65 does not con- MBNL1 and U2AF65, but the fluorescence assay strongly stitute a novel mechanism, because other alternative splicing suggests that these proteins bind this RNA in different con- factors have been shown to compete directly with U2AF65. The formations (Fig. 5). Both the differential binding sites of alternative splicing factor Sex Lethal (Sxl) has been shown to MBNL1 and U2AF65, and the role that RNA secondary compete with U2AF65 in the tra and msl-2 pre-mRNAs (13, 14), structure can play in modulating U2AF65 binding, suggest that whereas the Polypyrimidine Tract Binding Protein (PTB) has these splicing factors compete for recognition of the 3Ј end of been observed to compete with U2AF65 for sequences within intron 4 largely through binding mutually exclusive RNA the ␣-or␤-tropomyosin pre-mRNAs (21, 22). structures (see Fig. 6 for model). Sxl and PTB both have binding specificities similar to that of U2AF65, because all 3 proteins primarily prefer long runs The Role of MBNL1 in Regulating Alternative Splicing by Modulating RNA Secondary Structure. It is well documented that RNA struc- tural elements alone can regulate alternative splicing. For in- Model of Regulation of cTNT Exon 5 stance, stable structures have been shown to inhibit U1 snRNP Pathway to Exon 5 Inclusion Pathway to Exon 5 Exclusion binding to the 5Ј-splice site following exon 7 of both the SMN1 and SMN2 pre-mRNAs (23). Other structural elements that A splicing factor binding MBNL1 binding blocks U2AF65 U2AF65 binding ␤ and splice site defintion MBNL1 X encompass exon 6B in chicken -tropomysin have been shown to Exon 4 Exon 5 also inhibit binding of all of the U snRNPs, promoting the U2AF65 binds the intron in a single-stranded structure U2AF65 OR skipping of that exon (24). At present, it is unclear whether these Stabilized stem-loop examples also involve alternative splicing factors that regulate Exon 4 Exon 5 X inhibits U2AF65 binding these RNA structures, as MBNL1 appears to regulate secondary Exon 4 Exon 5 structures within the cTNT pre-mRNA. B spliceosome recruitment no U2AF65 bound to recruit U2 to intron 4, Other alternative splicing factors have been postulated to and U2 only recruited to intron 5 U2 recruited to both U2 regulate RNA structural elements, but there is no clear evidence U2 introns 4 and 5 MBNL1 X to show that these factors modulate RNA structure in a way that Exon 4 Exon 5 Exon 6 Exon 4 Exon 5 Exon 6 directly regulates alternative splicing. For example, the splicing factor hnRNP A1 has been shown to inhibit binding of splicing C splicing of “defined” introns Intron 4, exon 5 and intron 5 spliced out together factors ASF/SF2 and SC35 when they regulate splicing of the tat Both intron 4 and 5 spliced out separately spliceosome spliceosome pre-mRNA from HIV-1 (25). Binding of hnRNP A1 is thought Exon 4 Exon 6 Exon 4 Exon 5 Exon 6 to alter the secondary structure of the pre-mRNA in a way that Exon 5 affects splicing (26), but this has not been demonstrated. Simi-

D splice product larly, ribosomal protein L32 from S. cerevisiae binds a structured RNA element near the 5Ј-splice site of an intron within its own Exon 4 Exon 5 Exon 6 Exon 4 Exon 6 pre-mRNA, leading to intron retention (27). However, this structured binding site still allows for recruitment of the U1 Fig. 6. Model of regulation of cTNT exon 5 by MBNL1 and RNA structure. (A) Initial recognition of intron 4 by splicing factors. U2AF65 binds the intron in a snRNP, showing that RNA structures do not always inhibit single-stranded structure, but U2AF65 binding is inhibited if it cannot desta- binding of the U snRNPs (28). It is still unclear how L32 causes bilize the stem because of MBNL1 binding or mutations that stabilize the intron retention if it does not inhibit binding of U1. stem-loop. (B) U2 snRNP recruitment to all 3Ј splice sites where U2AF65 is In regards to the competition between MBNL1 and U2AF65, present. (C) Spliceosomal recruitment and splicing. (D) mRNA splice products. it has been demonstrated with a crystal structure that U2AF65

4of6 ͉ www.pnas.org͞cgi͞doi͞10.1073͞pnas.0900342106 Warf et al. Downloaded by guest on October 3, 2021 binds the py-tract in a single-stranded structure (29). In this 1:1 interaction between the RNA and protein, which allows only an apparent structure, 7 uridine residues bind along 2 RNA recognition Kd to be determined for RNAs containing more than one binding site. To motifs of U2AF65, leaving the 3Ј and 5Ј ends spatially far from determine the standard error of the apparent Kds, 3–5 binding titrations were each other. This crystal structure sheds light on why U2AF65 performed with each substrate and the apparent Kd values determined for each titration separately, prior to averaging. The error bars on the binding must bind the cTNT intron in a single-stranded form, because it curve were obtained by averaging the individual titration points and calcu- is unlikely that the stem could form with the loop in such an lating the standard deviation. extended structure. At present, MBNL1 is an alternative splicing factor that has been shown to regulate alternative splicing on the Complex Formation Assay. HeLa nuclear extract was prepared (30) and com- level of RNA structure. It is likely that other examples will follow plex formation was performed (31) as previously described. In short, HeLa as it is becoming increasingly apparent that RNA structure plays extract was added to a solution with RNA and buffer and incubated at 30 °C important roles in controlling pre-mRNA splicing. for the appropriate time point. The sample volume was 12 ␮L, and contained 25% HeLa extract, 55 mM KOAc, 30 mM KCl, 14.5 mM Hepes (pH 8.0), 10% Materials and Methods glycerol, 0.12 mM EDTA (pH 8.0), 0.5 mM DTT, 3.2 mM MgCl2, 0.5 mM ATP, 20 ␮ Cloning and Protein Purification. MBNL1 and U2AF65 were purified as de- mM creatine phosphate, 0.7 units/ L of RNase Inhibitor (Ambion). After the ␮ scribed (5, 18), and are described in SI Text. appropriate time point, samples were incubated with 4 L of heparin (4 mg/mL), and incubated at room temp for 2 minutes. ADML and cTNT-wt samples were then loaded onto a prechilled 1.8% agarose gel (1x TG), and run RNA Synthesis, Labeling, and Purification. The RNA substrates used for the at 75 volts for 1.5 hours, at 4 °C, in a 1x TG solution. To better resolve A Complex formation assay were transcribed with T7 polymerase, off linearized complex, cTNT-4G samples had 80 mM KOAc, and were run on a 1.2% agarose Puc19 plasmid. During transcription, RNAs were radiolabeled using gel (1x TG), for 4.5 hours at 40 volts, 4 °C. The gels were then fixed in 10% acetic [␣-32P]CTP. All other RNA substrates were ordered from IDT DNA, and 5Ј acid, 10% methanol solution for 10 min. Gels were dried, autoradiographed, end-labeled using [␥-32P]ATP. and quantified using ImageQuant (Molecular Dynamics). To determine the fold reduction, the percentage of complex formed in the absence of MBNL1 Cross-Linking Assay. RNA and proteins were incubated at room temperature was divided by the percentage complex formed in the presence of MBNL1. for 10 min, then on ice for 10 min, under the same conditions as used in the gel shift assay. Samples were placed on a pre-chilled block and exposed to UV for 2-Amino Purine Fluorescence. RNA (Dharmacon RNA Technologies) was diluted 1 min 20 seconds (1 Joule/cm2), 3.5 cm from the light source using a FB-UVXL- to 1 ␮M concentration in the binding buffer (175 mM NaCl, 20 mM NaCl, 5 mM 1000 lamp (Fisher Scientific). Samples were then incubated in protein dena- MgCl ) and snap annealed. All reactions were performed at room tempera- turing buffer for 2 min at 95°C and were resolved on a 10% denaturing 2 ture, in the binding buffer, using an L-formate Jobin-Yvon Horiba Fluoromax SDS/PAGE gel. Gels were then dried and autoradiographed. fluorimeter.A3mmwide Spectrosil microcell cuvette (Starna Cells, Inc) was BIOCHEMISTRY used. The 2-amino purine was excited at 312 nm, and spectra were collected Gel Shift Assay. Final conditions for the RNA-U2AF65 binding experiments from 310 to 420 nm. The fluorimeter slits were 3 nm , with an integration time were 175 mM NaCl, 5 mM MgCl , 20 mM Tris (pH 7.5), 1.25 mM BME, 12.5% 2 of 0.2 seconds. Spectra collected with buffer and protein was used to subtract glycerol, 2 mg/ml BSA, and 0.1 mg/ml Heparin. Prior to incubation, RNA out background. Three spectra were collected and averaged for every titra- substrates were snap annealed in 66 mM NaCl, 6.7 mM MgCl , and 27 mM Tris 2 tion point, with 3–4 titrations performed to determine average values and (pH 7.5). Protein was then added to the RNA, in a 10 ␮L volume and incubated standard deviations. To calculate relative fluorescence, the value at 375 nm for 10 min at room temp before 3–5␮L were loaded on a prechilled 5% was used. acrylamide (37.5:1), 0.5x TB gel, and run for 1 hr at 175 volts, at 4 °C. Gels were dried and autoradiographed. In Vivo Splicing. HeLa cell transfections, and RT-PCR was done as described in For binding curves, gels were quantified using ImageQuant (Molecular ref. 5, and are described in the SI Text. Dynamics). The percentage of RNA bound was determined by taking the ratio of RNA:Protein complex to total RNA, per lane. Binding curves were graphed and apparent Kd values were determined with KaleidaGraph (Synergy) soft- ACKNOWLEDGMENTS. We thank members of the Berglund, von Hippel, and 2 ware using the following equation: y ϭ ((m2 ϩ m1 ϩ m0) Ϫ ((Ϫm2 Ϫ m1 Ϫ m0) Barkan labs (especially Kausiki Datta and Alice Barkan) for experimental 0.5 Ϫ (4 ϫ m1 ϫ m0)) /(2 ϫ m1), where y ϭ % RNA bound, m2 ϭ Kd,m1ϭ total advice and help with equipment. This work was supported by National Insti- RNA concentration and m0 ϭ protein concentration. This equation assumes a tutes of Health Grants AR053903 (to J.A.B.) and GM-15792 (to P.H.v.H.).

1. Garcia-Blanco MA, Baraniak AP, Lasda EL (2004) Alternative splicing in disease and 14. Valcarcel J, Singh R, Zamore PD, Green MR (1993) The protein Sex-lethal antagonizes therapy. Nature Biotech 22:535–546. the splicing factor U2AF to regulate alternative splicing of transformer pre-mRNA. 2. Ranum LP, Cooper TA (2006) RNA-mediated neuromuscular disorders. Annu Rev Nature 362:171–175. Neurosci 29:259–277. 15. Reed R (1990) Protein composition of mammalian spliceosomes assembled in vitro. 3. Cho DH, Tapscott SJ (2007) Myotonic dystrophy: Emerging mechanisms for DM1 and Proc Natl Acad Sci USA 87:8031–8035. DM2. Biochim Biophys Acta 1772:195–204. 16. Reed R, Griffith J, Maniatis T (1988) Purification and visualization of native spliceo- 4. Nykamp KR, Swanson MS (2004) Toxic RNA in the nucleus: Unstable microsatellite somes. Cell 53:949–961. expression in neuromuscular disease. Prog Mol Subcell Biol 35:57–77. 17. Datta K, Johnson NP, von Hippel PH (2006) Mapping the conformation of the nucleic 5. Warf MB, Berglund JA (2007) MBNL binds similar RNA structures in the CUG repeats of acid framework of the T7 RNA polymerase elongation complex in solution using myotonic dystrophy and its pre-mRNA substrate cardiac troponin T. RNA 13:2238– low-energy CD and fluorescence spectroscopy. J Mol Biol 360:800–813. 2251. 18. Henscheid KL, Voelker RB, Berglund JA (2008) Alternative modes of binding by U2AF65 6. Ho TH, et al. (2004) Muscleblind proteins regulate alternative splicing. EMBO J at the polypyrimidine tract. Biochemistry 47:449–459. 23:3103–3112. 19. Teplova M, Patel DJ (2008) Structural insights into RNA recognition by the alternative- 7. Yuan Y, et al. (2007) Muscleblind-like 1 interacts with RNA hairpins in splicing target splicing regulator muscleblind-like MBNL1. Nat Struct Mol Biol 15:1343–1351. and pathogenic RNAs. Nucleic Acids Res 35:5474–5486. 20. Yi-Brunozzi HY, Stephens OM, Beal PA (2001) Conformational changes that occur 8. Hino S, et al. (2007) Molecular mechanisms responsible for aberrant splicing of SERCA1 during an RNA-editing adenosine deamination reaction. J Biol Chem 276:37827– in myotonic dystrophy type 1. Hum Mol Genet 16:2834–2843. 37833. 9. Penalva LO, Sanchez L (2003) RNA binding protein sex-lethal (Sxl) and control of 21. Singh R, Valcarcel J, Green MR (1995) Distinct binding specificities and functions of Drosophila sex determination and dosage compensation. Microbiol Mol Biol Rev higher eukaryotic polypyrimidine tract-binding proteins. Science 268:1173–1176. 67:343–359, table of contents. 22. Lin CH, Patton JG (1995) Regulation of alternative 3Ј splice site selection by constitutive 10. Gaur RK, Valcarcel J, Green MR (1995) Sequential recognition of the pre-mRNA branch splicing factors. RNA 1:234–245. point by U2AF65 and a novel spliceosome-associated 28-kDa protein. RNA 1:407–417. 23. Singh NN, Singh RN, Androphy EJ (2007) Modulating role of RNA structure in alterna- 11. Ruskin B, Zamore PD, Green MR (1988) A factor, U2AF, is required for U2 snRNP binding tive splicing of a critical exon in the spinal muscular atrophy genes. Nucleic Acids Res and splicing complex assembly. Cell 52:207–219. 35:371–389. 12. Valcarcel J, Gaur RK, Singh R, Green MR (1996) Interaction of U2AF65 RS region with 24. Sirand-Pugnet P, Durosay P, Clouet d’Orval BC, Brody E, Marie J (1995) beta- pre-mRNA branch point and promotion of base pairing with U2 snRNA [corrected]. Tropomyosin pre-mRNA folding around a muscle-specific exon interferes with several Science 273:1706–1709. steps of spliceosome assembly. J Mol Biol 251:591–602. 13. Merendino L, Guth S, Bilbao D, Martinez C, Valcarcel J (1999) Inhibition of msl-2 splicing 25. Zhu J, Mayeda A, Krainer AR (2001) Exon identity established through differential by Sex-lethal reveals interaction between U2AF35 and the 3Ј splice site AG. Nature antagonism between exonic splicing silencer-bound hnRNP A1 and enhancer-bound 402:838–841. SR proteins. Mol Cell 8:1351–1361.

Warf et al. PNAS Early Edition ͉ 5of6 Downloaded by guest on October 3, 2021 26. Damgaard CK, Tange TO, Kjems J (2002) hnRNP A1 controls HIV-1 mRNA splicing 29. Sickmier EA, et al. (2006) Structural basis for polypyrimidine tract recognition by the through cooperative binding to intron and exon splicing silencers in the context of a essential pre-mRNA splicing factor U2AF65. Mol Cell 23:49–59. conserved secondary structure. RNA 8:1401–1415. 30. Krainer AR, Maniatis T, Ruskin B, Green MR (1984) Normal and mutant human 27. Eng FJ, Warner JR (1991) Structural basis for the regulation of splicing of a yeast beta-globin pre-mRNAs are faithfully and efficiently spliced in vitro. Cell 36:993– messenger RNA. Cell 65:797–804. 1005. 28. Vilardell J, Warner JR (1994) Regulation of splicing at an intermediate step in the 31. Das R, Reed R (1999) Resolution of the mammalian E complex and the ATP-dependent formation of the spliceosome. Genes Dev 8:211–220. spliceosomal complexes on native agarose mini-gels. RNA 5:1504–1508.

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