Investigations on the maturation and regulation of archaebacterial proteasomes

The 20 S proteasome (core particle, CP) is a multifunctional protease complex and composed of four heptameric subunit rings arranged in a hollow, barrel-shaped structure. Here, we report the crystal structure of the CP from Archaeoglobus fulgidus at 2.25 Å resolution. The analysis of the structure of early and late assembly intermediates of this CP gives new insights in the maturation of archaebacterial CPs and indicates similarities to assembly intermediates observed in eukaryotes. We also show a striking difference in mechanism and regulation of substrate access between eukaryotic and archaebacterial 20 S proteasomes. While eukaryotic CPs are auto-inhibited by the N-terminal tails of the outer α-ring by imposing topological closure with a characteristic sequence motif (YDR-motif) and show regulatory gating this segment is disordered in the CP and differently structured in the α₇-sub-complex of A. fulgidus leaving a pore leading into the particle with a diameter of 13 Å. Mutagenesis and functional studies indicate the absence of regulatory gating in the archaeal 20 S proteasome.