Enzymatic cross-linking of involucrin and other proteins by keratinocyte particulates in vitro

A transglutaminase-catalyzed cross-linking process characteristic of keratinocytes leads to the formation of the insoluble corneocyte envelope. The essentials of this process take place in vitro in a reconstituted system derived from subcellular fractions. A particulate fraction containing membrane-bound envelope precursor proteins and the enzyme transglutaminase is combined with cytosolic proteins; when the enzyme is activated by Ca⁺⁺, cytosolic proteins are removed from solution and cross-linked to particulate proteins. This interaction is cell-type-specific, since particulates derived from fibroblasts and also containing transglutaminase activity cannot substitute for those of keratinocytes. Involucrin, a cytosolic protein known to be a precursor of the envelope, is more efficiently cross-linked than other cytosolic proteins. The cross-linking of proteins of the particulate fraction (membrane proteins) is promoted by the presence of involucrin.