Conformational Studies of the Par-4 C-Terminal Domain

Par-4 is an approximately 40 kilodalton tumor suppressor protein that is critical for apoptotic processes that affect a wide variety of cancer cells. Full-length Par-4 is known to self-associate via its C-terminal domain, although it has also been shown to be largely disordered in vitro. However, various isolated regions of Par-4 have been shown to contain structure. The C-terminal quarter of Par-4 consists of a coiled coil that includes a leucine zipper as its C-terminal half. The isolated leucine zipper forms a stable helical dimer in vitro only under conditions of either low pH or high salt concentration. The reason for this requirement has been traced to a negative-negative charge interaction across the dimer interface. The cl-Par-4 fragment, which comprises the C-terminal half of Par-4, has also been studied. Again, a high degree of helicity is evident in the presence of low pH or high salt, though the self-association state differs between an apparent coiled coil dimer and a non-coiled tetramer, respectively, under these conditions. These results also suggest that the SAC domain, which is the minimal region required for triggering apoptosis selectively in cancer cells, may be disordered in FL-Par-4 but ordered in cl-Par-4. Possible implications for apoptotic processes are discussed.