Abstract
Degradation by proteasomes involves coupled translocation and unfolding of its protein substrates. Six distinct but paralogous proteasome ATPase proteins, Rpt1 to -6, form a heterohexameric ring that acts on substrates. An axially positioned loop (Ar-Φ loop) moves in concert with ATP hydrolysis, engages substrate, and propels it into a proteolytic chamber. The aromatic (Ar) residue of the Ar-Φ 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 phenotypes. 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 phenotype probably 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-Φ 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.
Highlights
Proteasomes have six non-identical ATPases that move and unfold protein substrates
The strength of effect is scored semiquantitatively with respect to wild type. 0 is used to represent phenotype or activity similar to wild type, and the number of positive or negative signs represents the extent of deviation from wild type
Mutants are tabulated in an order that draws attention to their functional clustering
Summary
Results: Mutating homologous substrate contact residues of each ATPase has diverse effects on degradation and cell growth. The rpt mutant grew faster than wild type and to a smaller size, a defect that has previously been associated with misregulation of G1 cyclins. This rpt phenotype probably results from altered degradation of cell cycle regulatory proteins. Mutation of five of the Rpt subunits increased proteasome ATPase activity, implying bidirectional coupling between the Ar-⌽ loop and the ATP hydrolysis site.
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