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The deposition of protein aggregates in neurons is a hallmark of neurodegenerative diseases caused by polyglutamine (polyQ) proteins. We analyzed the effects of the heat shock protein (Hsp) 70 chaperone system on the aggregation of fragments of huntingtin (htt) with expanded polyQ tracts. In vitro, Hsp70 and its cochaperone Hsp40 suppressed the assembly of htt into detergent-insoluble amyloid-like fibrils in an ATP-dependent manner and caused the formation of amorphous, detergent-soluble aggregates. The chaperones were most active in preventing fibrillization when added during the lag phase of the polymerization reaction. Similarly, coexpression of Hsp70 or Hsp40 with htt in yeast inhibited the formation of large, detergent-insoluble polyQ aggregates, resulting in the accumulation of detergent-soluble inclusions. Thus, the recently established potency of Hsp70 and Hsp40 to repress polyQ-induced neurodegeneration may be based on the ability of these chaperones to shield toxic forms of polyQ proteins and to direct them into nontoxic aggregates.

The yeast two-hybrid system is a powerful technique that detects interactions between two proteins and has been useful in identifying new binding partners. However, the system fails to detect protein-protein interactions that require the presence of additional components of a multisubunit complex. Here we demonstrate that the vector YIpDCE1 can be used to express elongins B and C in yeast, and that these proteins form a stable complex that interacts with the von Hippel-Lindau tumor-suppressor gene product (pVHL). Only when pVHL and elongins B and C (VBC) are present does an interaction with the cullin family member, hCUL-2, occur, forming the heterotetrameric pVHL/elongin BC/hCUL-2 complex. This system was then used to map the binding region of hCUL-2 for the VBC complex. The first amino-terminal 108 aa of hCUL-2 are necessary for interaction with the VBC complex. The elongin BC dimer acts as a bridge between pVHL and hCUL-2 because pVHL and hCUL-2 can form distinct complexes with elongins B and C. These results reveal a striking structural resemblance of pVHL/elongin BC/hCUL-2 complex with the E3-like ubiquitin ligase complex SKP1/Cullin/F-box protein with respect to protein composition and sites of interactions. Thus, it seems possible that pVHL/elongin BC/hCUL-2 complex will possess ubiquitin ligase activity targeting specific proteins for degradation by the proteasome.