Purification and molecular characterization of a secretory transglutaminase from coagulating gland of the rat.

A transglutaminase (TGase, EC 2.3.2.13) was isolated from the secretion of rat coagulating gland (CGS-TGase). The protein consists of a single polypeptide chain and has a molecular mass of 65 kDa. During purification the net charge changes from pI 7.6 in the crude extract to pI 8.5-8.7 for the purified protein. Nearly equal numbers of glutamyl- and lysyl-residues were detected by amino acid analysis. The protein therefore represents an appropriate substrate of autocatalytic crosslinking. The total number of cysteine residues is 18-19, six of which being present in free form. One of the thiol groups is essential for the enzymic activity. The protein core is glycosylated with mannosyl residues and in addition substituted with saturated acyl residues and phosphoinositol. The phosphoinositol anchor was demonstrated by use of a specific antibody. Removal of the acyl- and glycosyl-residues or of the total anchor group results in autoaggregation and decrease of enzymic activity. In contrast to tissue-type TGases, Ca2+ dependent enzymic activity of CGS-TGase is not inhibited by GTP. The secretory TGase shows no immunological cross-reactivity to tissue-type enzyme or blood factor XIII.