Tissue transglutaminase: From biological glue to cell survival cues

Tissue transglutaminase (TG2, EC 2.3.2.13) is a ubiquitous enzyme that catalyzes Ca²⁺-dependent post-translational modification of proteins by inserting highly stable (epsilon-[gamma-glutamy1] lysine) isopeptide bonds or by conjugating polyamines at selected peptide-bound glutamine residues. The TG2-catalyzed cross-linked products (generally high molecular mass scaffold of proteins) are of great physiological significance; they are highly stable and resistant to mechanical, chemical and proteolytic degradation. The accumulation of isopeptide bonds can be observed in skin, hair and during blood clotting and wound healing. In addition to transamidation activity, TG2 also exhibits GTPase activity and in response to certain agonist hormones can serve as a signal transducing G protein. Although predominantly a cytosolic protein, TG2 can translocate to the nucleus with the help of importin-alpha-3 protein or to the membranes in association with integrins. Moreover, TG2 can also be secreted outside the cell (by yet unknown mechanism) where it crosslinks proteins of the extracellular matrix (ECM) and promotes cell adhesion and spreading. Another important property of TG2 is that it has high binding-affinity for the ECM component protein, fibronectin and thus can promote interaction between cell surface integrin with fibronectin. In this review, we discuss the implications of increased TG2 expression in drug-resistant and metastatic cancer cells and that how TG2 expression can contribute in the development of these phenotypes.