Crystal structure of a chaperone complex that contributes to the assembly of yeast 20S proteasomes

Eukaryotic 20S proteasomes are composed of two α-rings and two β-rings, which form an αββα stacked structure. Here we describe a proteasome-specific chaperone complex, designated Dmp1–Dmp2, in budding yeast. Dmp1–Dmp2 directly bound to the α5 subunit to facilitate α-ring formation. In Δdmp1 cells, α-rings lacking α4 and decreased formation of 20S proteasomes were observed. Dmp1–Dmp2 interacted with proteasome precursors early during proteasome assembly and dissociated from the precursors before the formation of half-proteasomes. Notably, the crystallographic structures of Dmp1 and Dmp2 closely resemble that of PAC3—a mammalian proteasome-assembling chaperone; nonetheless, neither Dmp1 nor Dmp2 showed obvious sequence similarity to PAC3. The structure of the Dmp1–Dmp2–α5 complex reveals how this chaperone functions in proteasome assembly and why it dissociates from proteasome precursors before the β-rings are assembled.