Transglutaminase-like superfamily. This family includes animal transglutaminases and other bacterial proteins of unknown function. Sequence conservation in this superfamily primarily involves three motifs that centre around conserved cysteine, histidine, and aspartate residues that form the catalytic triad in the structurally characterized transglutaminase, the human blood clotting factor XIIIa'. On the basis of the experimentally demonstrated activity of the Methanobacterium phage pseudomurein endoisopeptidase, it is proposed that many, if not all, microbial homologs of the transglutaminases are proteases and that the eukaryotic transglutaminases have evolved from an ancestral protease.

Transglutaminase/protease-like homologues. Transglutaminases are enzymes that establish covalent links between proteins. A subset of transglutaminase homologues appear to catalyse the reverse reaction, the hydrolysis of peptide bonds. Proteins with this domain are both extracellular and intracellular, and it is likely that the eukaryotic intracellular proteins are involved in signalling events.