Catalysis of the transfer of URM1 to a substrate protein via the reaction X-URM1 + S = X + S-URM1, where X is either an E2 or E3 enzyme, the X-URM1 linkage is a thioester bond, and the S-URM1 linkage is an isopeptide bond between the C-terminal amino acid of URM1 and the epsilon-amino group of lysine residues in the substrate.
E3