Transglutaminases (EC 2.3.2.13) catalyze the post-translational modifications of proteins by the formation of{var_epsilon}({gamma}-glutamyl)lysine insopeptide bonds. The number of transglutaminases recently described in eukaryotic cells includes at least five distinct enzymes that have been localized both intracellularly (tissue transglutaminase, keratinocyte transglutaminase, hair follicle transglutaminase) and extracellularly (factor XIIIa and prostate transglutaminase). Although these enzymes share some common features, such as the amino acid sequence in the active site and a strict calcium dependence for their catalytic activity, many differences in the biochemical and immunological properties have been described among the different members of this family. These findings suggest that each molecular form of transglutaminase, catalyzing the cross-linking of specific substrate proteins in specific biological districts, plays a different role in various physiological processes (i.e., blood coagulation, keratinocyte terminal differentiation, cell differentiation, apoptosis, seminal fluid coagulation, and sperm immunogenicity suppression). 10 refs., 1 fig.