Structural landscape of AAA+ ATPase motor states in the substrate-degrading human 26S proteasome reveals conformation-specific binding of TXNL1

Abstract

The 26S proteasome targets many cellular proteins for degradation during general homeostasis, protein quality control, and the regulation of vital processes. A broad range of proteasome-interacting cofactors thereby modulates these functions and aids in substrate degradation. Here, we solved several high-resolution structures of the redox active cofactor TXNL1 bound to the human 26S proteasome at saturating and sub-stoichiometric concentrations by time resolved cryo-EM. We identified distinct binding modes of TXNL1 that depend on the proteasome conformational and ATPase motor states. Together with biophysical and biochemical experiments, our structural studies reveal that the resting-state proteasome prior to substrate engagement with the ATPase motor binds TXNL1 with low affinity and in variable positions on top of the Rpn11 deubiquitinase. In contrast, the actively degrading proteasome shows additional interactions leading to high-affinity TXNL1 binding, whereby TXNL1s C-terminal tail covers the catalytic groove of the Rpn11 deubiquitinase and coordinates the active-site Zn2+. Furthermore, these cryo-EM structures of the degrading proteasome capture the ATPase hexamer in all registers of spiral-staircase arrangements and thus visualize the complete ATP-hydrolysis cycle of the AAA+ motor, indicating temporally asymmetric hydrolysis and conformational changes in bursts during mechanical substrate unfolding and translocation. Remarkably, we catch the proteasome in the act of unfolding the beta-barrel mEos3.2 substrate while the ATPase hexamer is in a particular spiral staircase register. Our findings challenge current models for protein translocation through hexameric AAA+ motors and reveal how the proteasome uses its distinct but broad range of conformational states to coordinate cofactor binding and substrate processing. HighlightsO_LIHigh resolution structures of the redox active cofactor TXNL1 in complex with the human 26S proteasome solved by time-resolved cryo-EM. C_LIO_LITXNL1 binds the catalytic groove of the main proteasomal deubiquitinase Rpn11 and coordinates its active-site Zinc specifically in substrate-degrading states of the proteasome. C_LIO_LIVisualizing a partially unfolded intermediate of the mEos model substrate during processing. C_LIO_LIStructures of the actively degrading human proteasome reveal all spiral-staircase registers of the AAA+ ATPase hexamer with unexpected nucleotide occupancies that indicate asymmetric ATP hydrolysis mechanisms, conformational changes with burst phases, and thus new models for hand-over-hand substrate translocation. C_LI