Evolutionary conservation of ubiquitin-like protein urmylation as revealed by URM1 gene shuffle from archaea to yeast

Urm1 from yeast is a unique ubiquitin-like protein with dual functionality. It has been shown to operate in tRNA thiolation and protein urmylation, combining features typical of bacterial sulfur carriers and classical ubiquitin-like modifiers. Hence, in evolutionary terms, Urm1 may be placed at the crossroad of prokaryotic sulfur transfer and eukaryotic protein conjugation pathways. Prompted by Urm1-like proteins identified in Archaea, we examined Urm1 functional conservation using URM1 gene shuffle from Sulfolobus acidocaldarius to Saccharomyces cerevisiae. We find that archaeal Urm1 conjugates to peroxiredoxin Ahp1, a bona fide urmylation target in yeast, but cannot support tRNA thiolation. Ahp1 conjugation requires sulfur transfer onto the archaeal Urm1 modifier from Uba4, the E1-like urmylation activator in yeast. Thus, thioactivation of archaeal Urm1 and urmylation-like conjugation are conserved and exchangeable processes between Sulfolobus and Saccharomyces. Our survey underlines that Urm1 likely occupies a key role in the evolution of the ubiquitin-like protein family.