Catalyzes the calcium-dependent formation of isopeptide cross-links between glutamine and lysine residues in various proteins, as well as the conjugation of polyamines to proteins. Involved in the formation of the cornified envelope (CE), a specialized component consisting of covalent cross-links of proteins beneath the plasma membrane of terminally differentiated keratinocytes. Catalyzes small proline-rich proteins (SPRR1 and SPRR2) and LOR cross-linking to form small interchain oligomers, which are further cross-linked by TGM1 onto the growing CE scaffold (By similarity). In hair follicles, involved in cross-linking structural proteins to hardening the inner root sheath. L-glutaminyl-[protein] + L-lysyl-[protein] = [protein]-L-lysyl-N(6)-5-L-glutamyl-[protein] + NH4(+) Binds 3 Ca(2+) cations per subunit. Binds 1 Ca(2+) as a zymogen, and binds 2 more Ca(2+) cations, or other divalent metal cations, after proteolytic processing. Consists of two polypeptide chains, which are synthesized as a precursor form of a single polypeptide. Expressed in skin and stomach and, at lower levels, in testis, kidney and spleen (at protein level). On the basis of its catalytic activity, detected in the epidermis, around the granular and spinous layers but not in the outermost cornified layers. In hair follicles, mainly located in the medulla and the hair cortex. Expression starts at 11.5 dpc in the early two-layered epidermis. From 12.5 dpc, mainly expressed in the periderm cells and weakly in the epidermal basal cells. After epidermis keratinization, at 15.5 to 17.5 dpc, detected in the granular, cornified layers and in the hair follicle. Also expressed in heart, lung, bone, muscle, testis and blood vessels at 12.5, 13.5, 14.5 and 16.5 dpc, respectively. Activated by proteolytic processing. In vitro activation is commonly achieved by cleavage with dispase, a neutral bacterial protease. Physiological activation may be catalyzed by CTSL and, to a lesser extent, by CTSS (By similarity). Belongs to the transglutaminase superfamily. Transglutaminase family.