Despite the central role of the 26 S proteasome in eukaryotic cells, many facets of its structural organization and functioning are still poorly understood. To learn more about the interactions between its different subunits, as well as its possible functional partners in cells, we performed, with Marc Vidal's laboratory (Dana-Farber Cancer Institute, Boston, MA, U.S.A.), a systematic two-hybrid analysis using Caenorhaditis elegans 26 S proteasome subunits as baits [Davy, Bello, Thierry-Mieg, Vaglio, Hitti, Doucette-Stamm, Thierry-Mieg, Reboul, Boulton, Walhout et al. (2001) EMBO Rep. 2, 821–828]. A pair-wise matrix of all subunit combinations allowed us to detect numerous possible intra-complex interactions, among which some had already been reported by others and eight were novel. Interestingly, four new interactions were detected between two ATPases of the 19 S regulatory complex and three α-subunits of the 20 S proteolytic core. Possibly, these interactions participate in the association of these two complexes to form the 26 S proteasome. Proteasome subunit sequences were also used to screen a cDNA library to identify new interactors of the complex. Among the interactors found, most (58) have no clear connection to the proteasome, and could be either substrates or potential cofactors of this complex. Few interactors (7) could be directly or indirectly linked to proteolysis. The others (12) interacted with more than one proteasome subunit, forming ‘interaction clusters’ of potential biological interest.

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