Functional Asymmetries of Proteasome Translocase Pore

Abstract

Background: Proteasomes have six non-identical ATPases that move and unfold protein substrates. Results: Mutating homologous substrate contact residues of each ATPase has diverse effects on degradation and cell growth. Conclusion: Although homologous, each of the six has distinguishable functions. Significance: Structurally similar elements within a complex molecular machine can evolve distinct tasks, diversifying the range of accessible cellular responses.

ATP Binding by Proteasomal ATPases Regulates Cellular Assembly and Substrate-induced Functions of the 26 S Proteasome

“…For example, ATP binding mutations in individual yeast Rpt subunits produced a range of effects that vary according to the subunit (22). As with other features of proteasome function (43), these findings suggest non-redundant roles for Rpt subunits in ATP-dependent proteasome assembly. In contrast, a recent report indicated that ATP binding mutations of any Rpt subunit blocked 26 S proteasome assembly in mammalian cells (44).…”

Structural basis for dynamic regulation of the human 26S proteasome

“…S12). The contrasting sidechain patterns of the pore loops-the hydrophobic pore-1 versus the highly charged pore-2 loops-suggest that they might play distinct roles in processive substrate translocation (36,45). We speculate that the pore-1 loops may propel substrates forward along the AAA channel through hydrophobic interactions, whereas the pore-2 loops translate their interactions with substrates into enhancement of ATP hydrolysis (46), possibly by controlling the configuration of the conserved glutamate residue in the Walker B motif (39) (SI Appendix, Fig.…”

Slippery Substrates Impair ATP-dependent Protease Function by Slowing Unfolding

“…Since the unfolding ability or processivity of both ClpXP and the proteasome are reduced by low complexity sequences such as the GRR from p105 or the GAr from EBNA1, and in both cases the primary mechanism is through a reduction in the ability of the protease to unfold and degrade the substrate rather than through accelerated release, it is likely that the basic mechanism underlying processivity has been conserved between bacteria and eukaroytes despite the evolutionary distance between them. Indeed, the proteasome Rpt motor ring shares a set of so-called "aromatic paddles" with bacterial unfoldases such as ClpX and HslU, and these paddles are predicted to be important for grabbing on to elements of the substrate and translocating them through the central pore (23,24). Aromatic paddle mutations have been associated with failures of processivity (although the kinetic basis is not known), so it seems likely that substrate modifications that reduce processivity will do so through an interaction with the aromatic paddles (23).…”

“…Further experiments are necessary to determine how these paddles interact with the peptide chain and how these low complexity sequences affect said interaction. In the case of the proteasome, with six distinct ATPase subunits, it is also possible that some subunits are specialized for different sorts of pulling or holding interactions, or for pulling on different sorts of sequences (24,25).…”