Hepatocellular cytokeratins as substrates of transglutaminases

To examine whether hepatocellular cytokeratins can serve as substrates of transglutaminases (TG) TG-catalyzed incorporation of [3H]putrescine into, as well as cross-linking of, cytokeratins was studied. Purified guinea pig liver TG and mouse liver TG present in 105,000 x g supernatants were used as enzymes. Isolated mouse liver cytokeratin filaments, heterotypic tetramers (A2D2), as well as cytokeratin filaments reconstituted from isolated and column-purified liver cytokeratin polypeptides A and D served as substrates. Moreover, to more closely mimic the situation within the cell, mouse liver homogenate containing cytokeratins was also used. Cross-linked proteins were identified as cytokeratins by immunoblotting after sodium dodecyl sulfate-polyacrylamide gel electrophoresis using cytokeratin antibodies. The results indicated that mouse liver cytokeratins can serve as substrates of homologous and heterologous TG. However, liver cytokeratin components A and D differed in this respect. Depending on the experimental conditions either components D or components A were better substrates of TG-mediated cross-linking as revealed by increased high molecular weight aggregates, which failed to enter the gels, concomitant with a decrease of the monomer band suggesting a more intimate relationship between homologous cytokeratin polypeptides within the filament. The results presented provide the basis for studies of TG-induced cross-linking of cytoskeletal components in hepatocytes that may occur during liver cell injury associated with increased intracellular Ca2+ concentrations.