Biochemical characterisation of human transglutaminase 4

Abstract

Transglutaminases are protein modifying enzymes involved in physiological and pathological processes with potent therapeutic possibilities. Human TG4, also called prostate transglutami-nase, is frequently associated with pathological symptoms and particularly with cancer inva-siveness. Although rodent TG4 is well characterised, biochemical characteristics of human TG4 that could help the understanding of its way of action are not published. First, we analysed proteomics databases and found that TG4 protein is present in human tissues beyond the pros-tate. Then, we studied in vitro the transamidase activity of human TG4 and its regulation using the microtiter plate method. Human TG4 has low transamidase activity which prefers slightly acidic pH and a reducing environment. It is enhanced by submicellar concentrations of SDS suggesting that membrane proximity is an important regulatory event. Human TG4 does not bind GTP as tested by GTP-agarose and BODIPY-FL-GTPγS binding, and its proteolytic activation by dispase or when expressed in AD-293 cells was not observed either. We identified several potential hu-man TG4 glutamine donor substrates in the AD-293 cell extract by biotin-pentylamine incorpo-ration and mass spectrometry. Several of these potential substrates are involved in cell-cell in-teraction, adhesion and proliferation, suggesting that human TG4 could become an anticancer therapeutic target.