May function as a protein modifier covalently attached to lysine residues of substrate proteins. This may serve to target the modified proteins for degradation by proteasomes. May be modified by deamidation of its C-terminal glutamine to glutamate by the adjacently encoded deamidase. This could be a prerequisite to the subsequent conjugation, as shown in the other prokaryotic ubiquitin-like protein Pup. Belongs to the ubiquitin-like protein UBact family.