Factor XIII is activated by thrombin and calcium ion to a transglutaminase that catalyzes the formation of gamma-glutamyl-epsilon-lysine cross-links between fibrin chains, thus stabilizing the fibrin clot. Also cross-link alpha-2-plasmin inhibitor, or fibronectin, to the alpha chains of fibrin. L-glutaminyl-[protein] + L-lysyl-[protein] = [protein]-L-lysyl-N(6)-5-L-glutamyl-[protein] + NH4(+) Binds 1 Ca(2+) ion per subunit. Tetramer of two A chains (F13A1) and two B (F13B) chains. Secreted into the blood plasma. Cytoplasmic in most tissues, but also secreted in the blood plasma. The activation peptide is released by thrombin. There are four main allelic forms of this protein; F13A*1A, F13A*1B, F13A*2A and F13A*2B. In addition two other intermediate forms (F13A*(2)A and F13A*(2)B) seem to exist. The sequence shown is that of F13A*1B. The disease is caused by variants affecting the gene represented in this entry. Belongs to the transglutaminase superfamily. Transglutaminase family. Factor XIII entry