The human brm protein is cleaved during apoptosis: The role of cathepsin G

The human brm (hbrm) protein (homologue of the Drosophila melanogaster brah ma and Saccharomyces cervisiae SNF-2 proteins) is part of a polypeptide com plex believed to regulate chromatin conformation. We have shown that the hb rm protein is cleaved in NB4 leukemic cells after induction of apoptosis by UV-irradiation, DNA damaging agents, or staurosporine, Because hbrm is fou nd only in the nucleus, we have investigated the nature of the proteases th at may regulate the degradation of this protein during apoptosis. In an in vitro assay, the hbrm protein could not be cleaved by caspase-3, -7, or -6, the "effector" caspases generally believed to carry out the cleavage of nu clear protein substrates. In contrast, we find that cathepsin G, a granule enzyme found in NB4 cells, cleaves hbrm in a pattern similar to that observ ed in vivo during apoptosis. In addition, a peptide inhibitor of cathepsin C blocks hbrm cleavage during apoptosis but does not block activation of ca spases or cleavage of the nuclear protein polyADP ribose polymerase (PARP). Although localized in granules and in the Golgi complex in untreated cells , cathepsin G becomes diffusely distributed during apoptosis. Cleavage by c athepsin C removes a 20-kDa fragment containing a bromodomain from the carb oxyl terminus of hbrm, This cleavage disrupts the association between hbrm and the nuclear matrix; the 160-kDa hbrm cleavage fragment is less tightly associated with the nuclear matrix than full-length hbrm.