Purification and characterization of a cytosolic transglutaminase from a cultured human tumour-cell line

Transglutaminases are a family of Ca²⁺-dependent enzymes that catalyse the formation of isopeptide bonds between the side chains of glutamine and lysine residues. The enzymes have been hypothesized to be involved in a wide range of cellular processes, including growth and differentiation and stabilization of the cytoskeleton. The human epidermal carcinoma-cell line, A431 cells, have relatively high amounts of a cytosolic transglutaminase activity that varies upon treatment of the cells with epidermal growth factor. We demonstrate here that this cytosolic activity has the biochemical and immunological properties of a tissue transglutaminase. We also report the purification of this enzyme to apparent homogeneity by a protocol which involves a novel affinity-elution step. Polyclonal antibodies to the transglutaminase were raised and used to identify the enzyme by Western blotting. The availability of purified transglutaminase and anti-transglutaminase antibodies will permit further study of the role of this enzyme in the growth of this hormone-responsive human tumour-cell line.