Crystal Structures of the Rhodococcus Proteasome with and without its Pro-peptides: Implications for the Role of the Pro-peptide in Proteasome Assembly

To understand the role of the pro-peptide in proteasome assembly, we have determined structures of the Rhodococcus proteasome and a mutant form that prevents the autocatalytic removal of its pro-peptides. The structures reveal that the pro-peptide acts as an assembly-promoting factor by linking its own β-subunit to two adjacent α-subunits, thereby providing a molecular explanation for the observed kinetics of proteasome assembly. The Rhodococcus proteasome has been found to have a substantially smaller contact region between α-subunits compared to those regions in the proteasomes of Thermoplasma, yeast, and mammalian cells, suggesting that a smaller contact area between α-subunits is likely the structural basis for the Rhodococcus α-subunits not assembling into α-rings when expressed alone. Analysis of all available β-subunit structures shows that the contact area between β-subunits within a β-ring is not sufficient for β-ring self-assembly without the additional contact provided by the α-ring. This appears to be a fail-safe mechanism ensuring that the active sites on the β-subunits are activated only after proteasome assembly is complete.