S100 proteins are a family of 10-14 kDa EF-hand-containing calcium binding proteins that function to transmit calcium-dependent cell regulatory signals. S100 proteins have no intrinsic enzyme activity but bind in a calcium-dependent manner to target proteins to modulate target protein function. Transglutaminases are enzymes that catalyze the formation of covalent epsilon-(gamma-glutamyl)lysine bonds between protein-bound glutamine and lysine residues. In the present study we show that transglutaminase-dependent covalent modification is a property shared by several S100 proteins and that both type I and type II transglutaminases can modify S100 proteins. We further show that the reactive regions are at the solvent-exposed amino- and carboxyl-terminal ends of the protein, regions that specify S100 protein function. We suggest that transglutaminase-dependent modification is a general mechanism designed to regulate S100 protein function.

Human involucrin (hINV) is a constituent of the scaffolding of the cornified envelope. In the present study, we describe an in vitro model system to study the role of hINV in scaffold formation. We characterize the in vitro cross-linking of full-length (585 amino acid) recombinant hINV, rhINV(1–585). When reacted with detergent-solubilized, particulate transglutaminase type 1 (TG1) or partially purified type 2 transglutaminase (TG2), rhINV(1–585) functions as a TG substrate in a calcium-dependent manner. When the reaction is supplemented with ¹⁴C-putrescine tracer, the radiolabeled cosubstrate is incorporated into a high-molecular-weight product in a calcium-, rhINV(1–585)- and time-dependent manner. ³⁵S-rhINV(1–585) is also cross-linked to form a high-molecular-weight product. These results suggest that rhINV(1–585) is extensively multimerized. Products having a molecular weight smaller than authentic rhINV(1–585) are also formed, providing evidence for intramolecular cross-link formation. Transmission electron microscopy of cross-linked product reveals immunoreactive large-molecular-weight loop-string-loop and branched structures. Our studies (1) show that rhINV(1–585) is a substrate for both TG1 and TG2, (2) indicate that rhINV(1–585) can be cross-linked to form macromolecular products having distinct structural features, (3) demonstrate that rhINV(1–585) forms intramolecular cross-links when hINV concentration is limiting and (4) establish that hINV possesses reactive Gln and Lys residues.

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