We study how macromolecular complexes assemble, move, and function to perform important biological processes. We use a range of molecular biological and biochemical techniques, with a major focus on determining 3-D structures by X-ray crystallography and electron microscopy. Our interests include HIV, nucleosome remodeling and reorganization, proteasome activation, and molecular machines called AAA ATPases.
Some of our recent studies and consequent understanding of AAA ATPase mechanism is illustrated in the figure on the basis of our work on Vps4. The active Vps4 complex, whose structure is illustrated in the left-hand panel of the figure, comprises six Vps4 subunit, six dimers of the Vta1 cofactor protein, and a peptide substrate. The substrates are ESCRT-III proteins, which form filamentous assemblies that drive membrane fission for many membrane fission events, including the final steps of cell division and the budding of many viruses, including HIV. As shown in the middle panels, our structural and biochemical studies show how Vps4 binds to ESCRT-III by binding the side chains in an alternating series of type I and type II binding sites. This provides the key to understanding how this remarkable enzyme translocates ESCRT-III subunits through the pore of the Vps4 hexamer, thereby unfolding them and causing the ESCRT-III filaments to disassemble. These studies also explain how many other AAA ATPases function in a very wide range of essential biological processes. The mechanism is illustrated in the right-hand panel. Five of the Vps4 subunits form a helix that is stabilized by binding of ATP and matches the symmetry of an extended, unfolded polypeptide substrate. Hydrolysis of ATP at the bottom end of the Vps4 helix allows the subunit to move from the bottom end to release ADP, rebind ATP, and join the top end of the Vps4 helix. In doing so, the Vps4 hexamer walks along the ESCRT-III protein forcing it into an extended conformation, which in turn leads to the biological response.