W0394

Crystal Structures of the Rhodococcus Proteasomes : Implications for the Role of the Pro-Peptide in Proteasome Assembly. Young Do Kwon¹, István Nagy², Paul D. Adams³, Wolfgang Baumeister², and Bing K. Jap^{1, 4}, ¹Graduate group of Comparative Biochemistry, Univ. of California at Berkeley, Berkeley, CA 94720, ²Max-Planck-Inst. of Biochemistry, Am Klopferspitz 18a, D-82152 Martinsried, Germany, ³Physical Sciences Div., Lawrence Berkeley National Laboratory, Berkeley, CA 94720, ⁴Life Sciences Div., Lawrence Berkeley National Laboratory, Berkeley, CA 94720.

We have determined crystal structures of the wild-type and the mutant Rhodococcus proteasomes to understand the role of the pro-peptide in proteasome assembly. The structures reveal that the pro-peptide facilitates Rhodococcus proteasome assembly by providing an additional contact surface between two adjacent α-subunits and between α- and β-subunits. Our structures also show that contact regions between α-subunits in the Rhodococcus proteasome are substantially smaller than those regions in other known proteasomes; the smaller contact area between Rhodococcus α-subunits is likely the structural basis for the observed kinetics in which the Rhodococcus α-subunits do not assemble into α-rings when expressed alone. Analysis of all β-subunit proteasome structures also indicates that the contact areas between β-subunits within a β-ring are not sufficient for β-ring self-assembly without the additional contact provided by the α-ring. Thus, the interdependency of α- and β-subunits for proteasome assembly dictated by intersubunit contact surfaces appears to provide a fail-safe mechanism which prevents proteasomes from unregulated substrate degradation by coupling maturation of the active sites with completion of proteasome assembly.