A protein conjugation system in yeast with homology to biosynthetic enzymereaction of prokaryotes

Protein conjugation, such as ubiquitination, is the process by which the C- terminal glycine of a small modifier protein is covalently attached to targ et protein(s) through sequential reactions with an activating enzyme and co njugating enzymes. Here we report on a novel protein conjugation system in yeast. A newly identified ubiquitin related modifier, Urm1 is a 99-amino ac id protein terminated with glycine-glycine. Urm1 is conjugated to target pr oteins, which requires the C-terminal glycine of Urm1. At the first step of this reaction, Urm1 forms a thioester with a novel E1-like protein, Uba4. Delta urm1 and Delta uba4 cells showed a temperature-sensitive growth pheno type. Urm1 and Uba4 show similarity to prokaryotic proteins essential for m olybdopterin and thiamin biosynthesis, although the Urm1 system is not invo lved in these pathways. This is the fifth conjugation system in yeast, foll owing ubiquitin, Smt3, Rub1, and Apg12, but it is unique in respect to rela tion to prokaryotic enzyme systems. This fact may provide an important clue regarding evolution of protein conjugation systems in eukaryotic cells.