Abstract
DAP-kinase was initially identified as a gene whose anti-sense-mediated reduced expression protected HeLa cells from interferon-gamma-induced programmed cell death. It was cloned in our laboratory by a functional gene selection approach. According to its amino acid sequence, this 160 kDa protein was predicted to be a novel type of calmodulin-regulated serine/threonine kinase which carries ankyrin repeats and the death domain. In this work we have shown that the kinase was autophosphorylated and capable of phosphorylating an exogenous substrate in a Ca2+/calmodulin-dependent manner. We proved that calmodulin binds directly to the recombinant kinase, and generated a constitutively active kinase mutant by the deletion of the calmodulin-regulatory domain. By immunostaining and biochemical fractionations we demonstrated that the kinase is localized to the cytoskeleton, in association with the microfilament system, and mapped a region within the protein which is responsible for binding to the cytoskeleton. Several assays attributed a cell death function to the gene. Ectopic expression of wild-type DAP-kinase induced the death of target cells, and the killing property depended strictly on the status of the intrinsic kinase activity. Conversely, a catalytically inactive mutant that carried a lysine to alanine substitution within the kinase domain, displayed dominant-negative features and protected cells from interferon-gamma-induced cell death. DAP-kinase is therefore a novel cytoskeletal-associated cell death serine/threonine kinase whose activation by Ca2+/calmodulin may be linked to the biochemical mechanism underlying the cytoskeletal alterations that occur during cell death.