A sequence motif that is Similar to Ubiquitin (SUb) has been identified in the Saccharomyces cerevisiae ubiquitin-specific protease Ubp6. SUb is conserved in all known Ubp6 homologues from a spectrum of eukaryotic species and is also present in a group of hypothetical proteins of unknown function (Unk1-3) present in sequence databases. An N-terminal deletion mutant of Ubp6 that lacks SUb is still capable of cleaving α-linked ubiquitin fusions, suggesting that SUb forms a separate domain to the catalytic core of Ubp6 and demonstrating that it is not required for in vitro cleavage activity. A homology model of the 78 N-terminal amino acids of human Ubp6, based on the known fold of ubiquitin, is presented. In human Ubp6, SUb shares only 20% sequence identity with ubiquitin. Even weaker similarity occurs between S. cerevisiae SUb and ubiquitin. The homology model supports a ubiquitin-like fold for SUb and suggests that two conserved Lys residues, corresponding to Lys48 and Lys63 of ubiquitin, are functionally important.
