PROTEOLYTIC RELEASE OF KERATINOCYTE TRANSGLUTAMINASE

Human keratinocytes express a particulate transglutaminase that can be released from the membranes by limited proteolysis with trypsin or plasmin to yield a form that is ≃ 80 kDa. The enzyme from cultured cells was also releasable by endogenous proteolysis to yield a catalytically active fragment of ≃ 80 kDa. Endogenous release was strongly dependent upon temperature and Ca2+ concentration and was inhibited by iodoacetate, but not by leupeptin, antipain or phenylmethanesulphonyl fluoride. These phenomena raise the possibility of partial translocation of transglutaminase activity to the cytoplasm by proteolysis to which the enzyme is subject during terminal differentiation. In addition, hydrodynamic measurements showed that the endogenously released enzyme was monomeric in solution (79 kDa), whereas that solubilized by hydroxylamine without proteolysis appeared dimeric (190 kDa). The latter dimeric state may reflect either an altered conformation of the enzyme or post-translational modification beyond fatty acid esterification.