♦ See referenced article, J. Biol. Chem. 2012, 287, 12172–12182
The 26 S proteasome acts as the eukaryotic cell's garbage disposal, as it breaks down ubiquitinated proteins. The 2.5-MDa organelle contains a proteolytic core particle and two regulatory particles. The regulatory particles contain six ATPase molecules, among other things. There are chaperone proteins whose principal function is to assemble the 26 S proteasome, but the details are unknown. In this Paper of the Week, a team led by Tsunehiro Mizushima at the University of Hyogo and Yasushi Saeki at the Tokyo Metropolitan Institute of Medical Science studied crystal structures of Hsm3, a chaperone responsible for the assembly of the regulatory particles, both alone and in complex with the C-terminal domain of one of the six ATPase molecules, Rpt1. They discovered that the interacting surface between the two proteins has a hydrophobic core and a complementary charged surface. The investigators also found that Hsm3 may bind to another ATPase component, Rpt2. These results provide insight into how Hsm3 promotes the assembly of the regulatory particle of the 26 S proteasome.
Two orthogonal views of the crystal structure of Hsm3.