Urm1 is a ubiquitin related protein that modifies proteins in the yeast ubiquitin-like pathway urmylation [1]. Structural comparisons and phylogenetic analysis of the ubiquitin superfamily has indicated that Urm1 has the most conserved structural and sequence features of the common ancestor of the entire superfamily [2].

1: Solution structure of Urm1 and its implications for the origin of protein modifiers. Xu J, Zhang J, Wang L, Zhou J, Huang H, Wu J, Zhong Y, Shi Y; Proc Natl Acad Sci U S A. 2006;103:11625-11630. PMID:16864801

2: Urmylation: a ubiquitin-like pathway that functions during invasive growth and budding in yeast. Goehring AS, Rivers DM, Sprague GF Jr; Mol Biol Cell. 2003;14:4329-4341. PMID:14551258