西亚试剂：Functional asymmetries of the proteasome translocase

Functional asymmetries of the proteasome translocase pore

Jenny Erales, Martin A. Hoyt, Fabian Troll and Philip Coffino.

Degradation by proteasomes involves coupled translocation and unfolding of its protein substrates. Six distinct but paralogous proteasome ATPase proteins,Rpt1-6, form a heterohexameric ring which acts on substrates.An axially-positioned loop (Ar-Phi loop) moves in concert with ATP hydrolysis, engages substrate and propels it into a proteolytic chamber.The aromatic (Ar) residue of the Ar-Phi loop in all six Rpts of S. cerevisiae is tyrosine; this amino acid is thought to have important functional contacts with substrate. Six yeast strains were constructed and characterized in which Tyr was individually mutated to Ala.The mutant cells were viable and had distinct s. rpt3, rpt4 and rpt5 Tyr/Ala mutants, which cluster on one side of the ATPase hexamer, were substantially impaired in their capacity to degrade substrates.In contrast, rpt1, rpt2 and rpt6 mutants equaled or exceeded wild type in degradation activity. However rpt1 and rpt6 mutants had defects that limited cell growth or viability under conditions that stressed the ubiquitin proteasome system. In contrast, the rpt3 mutant grew faster than wild type and to a smaller size, a defect that has previously been associated with misregulation of G1 cyclins.This rpt3 likely results from altered degradation of cell cycle regulatory proteins.Finally, mutation of five of the Rpt subunits, increased proteasome ATPase activity, implying bidirectional coupling between the Ar-Phi loop and the ATP hydrolysis site.The present observations assign specific functions to individual Rpt proteins and provide insights into the diverse roles of the axial loops of individual proteasome ATPases