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Proline-Rich Domain and a Cysteine-Rich Domain

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Proline-Rich Domain and a Cysteine-Rich Domain Proc. Natl. Acad. Sci. USA Vol. 90, pp. 6829-6833, July 1993 Plant Biology A tobacco gene family for flower cell wall proteins with a proline-rich domain and a cysteine-rich domain HEN-MING WU, JITAO Zou, BRUCE MAY, QING Gu, AND ALICE Y. CHEUNG* Department of Biology, Yale University, P.O. Box 6666, New Haven, CT 06511 Communicated by J. E. Varner, March 29, 1993 ABSTRACT Flowering is known to be associated with the different cell types and at different developmental stages induction of many cell wall proteins. We report here five (8-10). A flower is morphologically complex and composed members of a tobacco gene family (CELP, Cys-rich extensin- of multiple tissue types (11). Flowering is known to stimulate like protein) whose mRNAs are found predominantly in flowers or induce the expression of several cell-wall protein genes and encode extensin-like Pro-rich proteins. CELP mRNAs (12-18). These extracellular matrix proteins may be involved accumulate most abundantly in vascular and epidermal tissues in the organization of the floral apex from a vegetative apex of floral organs. In the pistil, CELP mRNAs also accumulate or may fulfill special structural and functional requirements in a thin layer of cells between the transmitting tissue and the for flower cell walls. Some flowering-associated extracellular cortex of the style and in a surface layer of cells of the placenta matrix proteins are also stress-inducible or pathogen-related in the ovary. This unique accumulation pattern of CELP and may contribute to the overall defense system in flowers mRNAs in the pistil suggests a possible role in pollination and (12, 19, 20). fertilization processes. CELP genes encode a class of plant We report here the characterization of a family of tobacco extracellular matrix proteins that have several distinct struc- genes and cDNAs (CELPs, Cys-rich extensin-like proteins) tural features: a Pro-rich extensin-like domain with Xaa-Pro3_7 for a set of proteins that have an extensin-like domain.t motifs and Xaa-Pro doublets, a Cys-rich region, and a highly CELP mRNAs accumulate almost exclusively in the four charged C terminus. The extensin-like domains in these pro- teins differ significantly in their length and these differences floral organs-sepal, petal, stamen, and pistil. CELP mRNAs appear to be results of both long and short deletions within the accumulate in multiple cell types in each floral organ. They coding regions of their genes. Furthermore, the number of are abundant in the vascular bundles and the epidermis, charged amino acid residues in the C-terminal region varies except in the pistil where they are most abundant in the cells among the CELPs. These structural differences may contribute demarcating the transmitting tissue and the cortex and in the to functional versatility in the CELPs. On the other hand, the cell layer lining the placenta to which ovules are attached. Cys-rich domain is highly conserved among CELPs and the The CELPs have three distinct structural domains: a Pro-rich positions ofthe Cys residues are conserved, suggesting that this extensin-like domain, a Cys-rich domain, and a highly region may have a common functional role. The presence of a charged C terminus. These features make the CELPs struc- Pro-rich domain and a Cys-rich domain in these CELPs is turally distinct from tobacco extensin (21). The possible reminiscent of a dass of hydroxyproline-rich glycoproteins, functional roles for this class of extracellular matrix proteins solanaceous lectins, that are believed to be important in will be discussed in light of the available structural and cell-cell recognition. The structure of these CELPs indicates expression information. that they may be multifunctional and that their genes may have arisen from recombinational events. MATERIALS AND METHODS The primary walls of plant cells are thin and flexible but strong and can accommodate cell expansion and changes in cDNA and Genomic Library Construction and Screening. cell shape as cells grow and differentiate. Besides providing CELP cDNA clones were isolated from a tobacco floral structural integrity, cell walls are also the sites where cell- cDNA library as described (20). A A Dash II genomic N. cell interactions as well as interactions between plants and tabacum DNA library was made as described (22). CELP their environment take place (1-5). Cell-wall proteins are genomic clones were isolated from an unamplified library diverse and include many proteins that are believed to play using a 32P-labeled CELP-lc probe prepared by random structural roles and others that are involved in defense or priming (22). Hybridization and washes were carried out at other cellular and biochemical processes. The best charac- 68°C in buffers as described (15). terized cell wall protein genes are those for several classes of Nucleotide Sequence Analysis. Nucleotide sequence analy- hydroxyproline (Hyp)-rich glycoproteins (HRGPs) (2, 6). sis was carried out by the dideoxynucleotide sequencing One class includes the extensins, which typically have nu- method using double-stranded DNA and Sequenase Version merous Ser-Pro(Hyp)4 motifs throughout the entire protein. 2 (United States Biochemical) according to the manufactur- Another class encodes what are collectively known as Pro All the were (Hyp)-rich proteins, which have multiple copies of the pen- er's recommendations. nucleotide sequences tapeptide Val-Tyr-Lys-Pro-Pro or its variants. There are at determined on both strands of DNA. least two additional classes of glycosylated Pro (Hyp)-rich RNA Expression Analysis. RNA preparation, gel electro- proteins, arabinogalactan proteins and solanaceous lectins. phoresis, blot analysis, and in situ hybridizations have been Gly-rich cell wall protein genes have also been reported (7). described (20). Differential expression of cell wall protein genes may contribute to meeting the functional and physical demands on Abbreviations: CELP, Cys-rich extensin-like protein; Hyp, hy- droxyproline; HRGP, Hyp-rich glycoprotein. *To whom reprint requests should be addressed. The publication costs ofthis article were defrayed in part by page charge tThe sequences reported in this paper have been deposited in the payment. This article must therefore be hereby marked "advertisement" GenBank data base (accession nos. CELP-1 to CELP-5, L13439- in accordance with 18 U.S.C. §1734 solely to indicate this fact. L13443, respectively). 6829 Downloaded by guest on September 24, 2021 6830 Plant Biology: Wu et al. Proc. Natl. Acad. Sci. USA 90 (1993) RESULTS Table 1. Amino acid composition of CELP-1, -2, -3, -4, and -5, tobacco extensin, and potato lectin CELP-1 and CELP-lc, a Gene and Its cDNA for an Extensin- Like Pro-Rich Protein. CELP-lc, a cDNA derived from a % of total residues class of highly expressed flower mRNAs, was isolated from CELP a tobacco floral cDNA library. CELP-lc has an open reading Residue 1 2 3 4 5 TE* PLt frame corresponding to a 209-aa Pro-rich protein (Fig. 1). The first of four closely spaced Met residues (NT 37-39) (Fig. 1) Ala 4.78 5.1 6.66 5.69 7.45 1.88 3.70 is assumed to function as the initiation codon of the deduced Arg 6.22 5.61 3.63 3.79 2.48 0 0.82 CELP-1 protein since the 5' end of the mRNA is located at Asn 2.87 2.55 3.63 2.53 2.48 0.31 5.34 the predominant transcription initiation site of the CELP-1 Asp 3.82 4.08 4.24 3.16 3.72 0 gene at the thymidine denoted NT 1 (data not shown). Cyst 6.70 6.63 7.87 8.22 8.07 0 11.5 The most striking feature of the deduced CELP-1 protein 9.87 1Q Ill 1Lu IILt is in the 74-aa central region that contains a series of Pro-rich Gln 2.87 3.57 2.42 3.79 3.72 0.31 6.99 sequences. The Pro residues are distributed in seven Xaa- Glu 5.26 5.61 6.06 5.69 5.59 0.31 0.95 1.02 1.81 1.26 2.48 0.31 11.5 Pro3 (Xaa being Trp, Cys, or Ser) and 18 Xaa-Pro Gly 5motifs His 0.478 0.51 0 0.63 0 4.71 0 being Trp, Phe, Cys, Arg, or Gln). Overall, Pro doublets (X Ile 3.34 3.06 5.45 3.79 4.96 0 1.23 are CELP-1 protein but they residues 26.3% of the entire Leu 5.74 5.61 7.87 8.86 9.31 2.20 1.23 make up 66.2% of the Pro-rich domain (see Table 1). The Lys 5.26 5.10 4.84 6.32 4.96 12.26 3.70 Xaa-Pro35 motifs in the predicted CELP are similar to the Met 2.39 3.57 1.81 1.26 2.48 0.628 0.41 Ser-Pro4 repeats found in many plant-cell-wall HRGPs (4, 6). Phe 2.39 2.04 4.24 3.16 2.48 0.31 0 CELP-1, especially its C-terminal half, is relatively Cys-rich Pro§ 26.3 26 26 18.9 19.25 39.93 28.3 (Fig. 1 and Table 1). Moreover, 9 ofthe last 11 aa are charged. I 67.1 65.7 A Characterization ofthe CELP-1 gene revealed a 400-bp intron Ser 8.13 7.14 7.27 6.96 6.83 10.06 11.1 that interrupts Gln196 and Val'97 (data not shown), thus Thr 2.87 3.06 3.63 4.43 3.10 5.03 6.58 separating the region encoding the highly charged C terminus Trp 2.39 2.55 0.60 1.89 2.48 0 3.29 of the CELP-1 from the rest of the gene.
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