Structural basis of ubiquitin-independent PP1 complex disassembly by p97

Abstract

The AAA+ ATPase p97 (also called VCP, or Cdc48 in yeast) unfolds proteins and disassembles protein complexes in a myriad of cellular processes, but how a substrate complex needs to be loaded onto p97 by a dedicated substrate adapter and then disassembled by p97 has not been structurally visualized so far. Here we present cryo-EM structures of p97 in the process of disassembling a protein phosphatase-1 (PP1) complex by stripping off an inhibitory subunit. We show that PP1 and its partners SDS22 and inhibitor-3 (I3) bind to a peripheral N-domain of p97 via a direct contact between SDS22 and a groove in the N-domain. A density consistent with the SHP box of the p37 adapter binds to the same N-domain underneath the PP1 complex, while the p37-UBX domain is found on the adjacent N-domain. I3 is likely represented by three densities. One covers the PP1 catalytic site adjacent to SDS22, another is at the PP1 binding site for the RVXF motif in I3 pointing towards the p97 pore, and the third is a peptide threaded through the central channel of the spiral-shaped p97 hexamer. Our data show how p97 arranges a substrate complex between the N-domain and central channel, and then extracts one component by threading it through the channel to disassemble the complex.