Urm1 Couples Sulfur Transfer to Ubiquitin-Like Protein Function in Oxidative Stress

COMMENTARY Urm1 couples sulfur transfer to ubiquitin-like protein function in oxidative stress Matthew D. Petroskia, Guy S. Salvesenb, and Dieter A. Wolfa,1 aSignal Transduction Program and bProgram in Apoptosis and Cell Death Research, Sanford-Burnham Medical Research Institute, La Jolla, CA 92037 he posttranslational modification of proteins with ubiquitin and T ubiquitin-like proteins (collectively referred to as UBLs) has emerged as a major regulatory mechanism in eukaryotes. UBLs are characterized by a core β-grasp fold and an essential carboxy terminal glycine within a di-glycine motif (1). These features are also found in several prokaryotic sulfur carriers, sug- gestive of an evolutionary relationship to UBLs (2–5). A case in point is the UBL Urm1 (ubiquitin-related modifier 1). Urm1 is known to be conjugated to the peroxiredoxin Ahp1 (6), but its sequence and structure more closely resembles bacterial sulfur carrier proteins (7). In PNAS, work by Van der Veen et al. provides important insight into the mechanism of Urm1 conjugation that highlights its position as an ancient UBL in eukaryotes and solidifies its roles as a posttranslational protein modifier involved in oxidative stress response mechanisms (8). All UBLs require ATP-dependent acti- Fig. 1. Juxtaposition of Urm1 as a prokaryotic sulfur carrier and a eukaryotic UBL. The shaded areas vation through their cognate E1 enzyme indicate overlapping aspects of the pathways, illustrating that Urm1 seems to use features common to both ubiquitin and MoaD for its activation and conjugation. The Urm1–E2 intermediate indicated in before conjugation onto target proteins brackets is still hypothetical but may account for the hydroxylamine sensitivity of Urm1–protein con- (Fig. 1). After the initial formation of jugates formed in response to oxidative stress. a UBL-adenylate in the nucleotide binding pocket of the E1 through its carboxy terminal glycine, the UBL is transferred onto above for UBLs, ATP-dependent activa- Urm1 also functions as a UBL to co- an acceptor sulfhydryl—the E1 catalytic tion promotes the formation of an acyl valently modify proteins. Unlike other cysteine—to form a high-energy thiolester disulfide with MoeB and ThiS, respectively UBL systems, however, enzymes other intermediate (9, 10). The ultimate co- (Fig. 1). This leads to the formation of than an E1 have not been identified; thus, valent transfer of the UBL onto its target a thiocarboxylate that then provides sulfur it remained unclear how activated Urm1 is proteins involves additional specialized necessary for subsequent reactions. transferred onto proteins, the nature of enzymes—E2s (conjugating enzymes) and How is Urm1 activated? Whereas the these conjugates, and how residues on sometimes E3s (ligases)—which receive Urm1 E1 (Uba4 in budding yeast, MOCS3 these proteins are selected for modifica- the UBL through a trans-thiolation re- in humans) was originally proposed to tion. Although previous studies detected action using other active site cysteines, form a thiolester linkage to the carboxyl covalent Urm1 conjugates in budding thereby providing additional layers of terminus of Urm1 (7), more recent in vitro yeast, only a single substrate, the peroxir- specificity and regulation. UBLs are usu- studies determined that Urm1 forms an edoxin Ahp1, has been identified (6, 7). ally transferred, via their carboxyl termi- acyl disulfide through its E1, leading to the Genetic studies implicated Urm1 and ε nus, onto the -amine of lysine side chains formation of thiocarboxylated Urm1 (ref. Ahp1 in oxidative stress response mecha- fi of their substrates in a site-speci c manner 13; Fig. 1). This mechanism requires two nisms (6, 20). iso to form a covalent -peptide bond. cysteine residues: one that has sulfur- Van der Veen et al. significantly extend The discovery of Urm1 in 2000 relied on transferase activity and another with these observations by demonstrating that the observation that prokaryotic sulfur adenylyltransferase activity. This is not oxidative stress specifically induces the carrier proteins, such as MoaD and ThiS, entirely surprising, given that MOCS3 is formation of Urm1–protein conjugates in contain the characteristic UBL di-glycine also involved in the biosynthesis of Moco both yeast and human cells (8). How do motif and require ATP-dependent activa- in humans (13–15). Thus, these observation through a mechanism similar to E1 tions suggest that Urm1 activation is more enzymes (7) (Fig. 1). These proteins similar to that of a prokaryotic sulfur car- Author contributions: M.D.P., G.S.S., and D.A.W. wrote the function as sulfur donors in biosynthetic rier protein than that of a UBL. This paper. pathways, with MoaD involved in molyb- function has been linked to the down- The authors declare no conflict of interest. denum cofactor (Moco) synthesis and stream thiolation of certain tRNAs during See companion article on page 1763. fi ThiS in thiamine biosynthesis (2, 5, 11, 12). oxidative stress, where their modi cation 1To whom correspondence should be addressed. E-mail: In contrast to the mechanism described alters their decoding specificity (16–19). [email protected]. www.pnas.org/cgi/doi/10.1073/pnas.1019043108 PNAS | February 1, 2011 | vol. 108 | no. 5 | 1749–1750 Downloaded by guest on September 30, 2021 these conjugates form? In the absence of sumably, such E2s exist because, as the ifications characterized to date, Urm1 oxidative stress and under steady-state present study shows, a thiocarboxylate on modification is also likely to be reversible conditions, ∼60% of Urm1 is in its thio- the C terminus of eGFP is not sufficient through an as-of-yet unknown “deurmy- carboxylate form. Thus, Urm1 activation is to enable it to engage in conjugate for- lating” enzyme. necessary but not sufficient for its conju- mation. Likewise, an E2–Urm1 thiolester What are the functions of Urm1 mod- gation. Through a clever series of bio- intermediate could explain the hydroxyl- ifications? As a UBL, Urm1 is anticipated chemical experiments, the authors dem- amine sensitivity of protein urmylation. to alter a protein’s function, such as tar- fi onstrate that in vitro-generated Urm1 In addition to con rming that Ahp1 is geting it for degradation, changing its thiocarboxylate—but not Urm1 carboxyl- a Urm1 substrate in yeast (6), the authors — subcellular localization, modulating its in- ate is conjugated to proteins in HeLa teractions with other proteins, or inducing cell extracts only in the presence of the ’ conformational changes. On the basis of thiol oxidizer diamide. Intriguingly, con- Urm1 s function as a the subset of proteins analyzed by Van der jugate formation also seems to require sulfur carrier is directly a thiolester linkage because treatment of Veen et al. (8) and previous studies on cells with hydroxylamine, a cell-permeable coupled to its functions Ahp1 (6), Urm1 does not seem to have compound that disrupts thiolester link- a role in targeting proteins for degrada- ages, reduces oxidative-stress–dependent as a UBL. tion. Exactly why proteins involved in Urm1 conjugation. Because the conjugates Urm1 functions, such as its E1 MOCS3, are largely resistant to the reducing agent would be modified in response to oxidative hydroxylamine, Urm1 modification in- identified by mass spectrometry a variety stress is unclear, although it is tempting volves a covalent iso-peptide linkage, of human proteins modified in response to to speculate that these may prevent sub- which the authors confirm is through ly- oxidative stress (8). These include proteins sequent sulfur mobilization through Urm1 sine residues of target proteins. involved in Urm1 functions (MOCS3, (8). The observation that two deubiquity- Collectively, these experiments suggest ATPD3, and CTU2), the deubiquitylating lating enzymes are modified suggests that Urm1’s function as a sulfur carrier enzymes USP15 and USP47, proteins in- a connection between these posttransla- is directly coupled to its functions as volved in nuclear transport, and RNA- tional modification systems; however, a UBL, linking both of these to oxidative processing proteins. Several of these were the authors were unable to uncover any stress response. Moreover, they imply that studied in more detail, and in all cases changes in the activity of these enzymes in fi Urm1 modi cation involves a unique oxidative stress induced a molecular response to Urm1 modification (8). The “ ” weight shift of the protein consistent with noncanonical mechanism for a UBL. discovery that CAS (cellular apoptosis Formation of the Urm1 thiocarboxylate a single Urm1 modification. Thus, multi- susceptibility) protein is modified in the by its E1 allows for oxidative-stress– ple Urm1 attachments or Urm1 polymers cytosol suggests a potential role in pre- dependent sulfur transfer and presumably do not seem to be synthesized on proteins the formation of a Urm1 thiolester before in cells. These modifications seem to be venting its translocation to the nucleus (8). covalent transfer onto substrate lysine restricted to a specific lysine side chain, However, this remains unanswered, as residues. Given that Urm1 activation is but exactly how substrate and site speci- does the question of why this would be through an unusual mechanism not in- ficity are defined remains a significant important for oxidative stress response. volving a thiolester linkage on the E1 ac- unresolved question. Unlike most UBLs, Nevertheless, it is now abundantly clear tive site cysteine and other “canonical” Urm1 is not synthesized as a precursor that Urm1 is activated and conjugated E1s or E2s have not been identified, the that needs to be trimmed at the carboxyl onto proteins through a mechanism that mechanism that allows for specific urmy- terminus to reveal the essential glycine, emphasizes its roles both as a sulfur lation currently remains unresolved. Pre- but by analogy to all other UBL mod- transfer protein and as a UBL. 1. Hochstrasser M (2009) Origin and function of ubiqui- 9. Ciechanover A, Heller H, Katz-Etzion R, Hershko A 15.

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