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. Belongs to the transglutaminase superfamily. Transglutaminase family.