Rhodanese-Like Domain Protein UbaC and Its Role in Ubiquitin-Like Protein Modification and Sulfur Mobilization in Archaea.

Ubiquitin-like protein (Ubl) modification targets proteins for transient inactivation and/or proteasome-mediated degradation in archaea. Here the rhodanese-like domain (RHD) protein UbaC (HVO_1947) was found to copurify with the E1-like enzyme (UbaA) of the Ubl modification machinery in the archaeon Haloferax volcanii UbaC was shown to be important for Ubl ligation, particularly for the attachment of the Ubl SAMP2/3s to protein targets after exposure to oxidants (NaOCl, dimethyl sulfoxide [DMSO], and methionine sulfoxide [MetO]) and the proteasome inhibitor bortezomib. While UbaC was needed for ligation of the Ubl SAMP1 to MoaE (the large subunit of molybdopterin synthase), it was not important in the formation of oxidant-induced SAMP1 protein conjugates. Indicative of defects in sulfur relay, mutation of ubaC impaired molybdenum cofactor (Moco)-dependent DMSO reductase activity and cell survival at elevated temperature, suggesting a correlation with defects in the 2-thiolated state of wobble uridine tRNA. Overall, the archaeal stand-alone RHD UbaC has an important function in Ubl ligation and is associated with sulfur relay processes.IMPORTANCE Canonical E2 Ub/Ubl-conjugating enzymes are not conserved in the dual-function Ubl systems associated with protein modification and sulfur relay. Instead, the C-terminal RHDs of E1-RHD fusion proteins are the apparent E2 modules of these systems in eukaryotes. E1s that lack an RHD are common in archaea. Here we identified an RHD (UbaC) that serves as an apparent E2 analog with the E1-like UbaA in the dual-function Ubl sampylation system of archaea. Unlike the eukaryotic E1-RHD fusion, the archaeal RHD is a stand-alone protein. This new insight suggests that E1 function in Ubl pathways could be influenced by shifts in RHD abundance and/or competition with other protein partners in the cell.