The accessory adapters FAF1, FAF2, and UBXN7 accelerate proteasomal degradation by increasing prior p97-mediated substrate unfolding

TL;DR

This study reveals how accessory proteins help the p97 complex unfold proteins more efficiently for degradation by the proteasome.

Contribution

The study identifies how FAF1, FAF2, and UBXN7 enhance p97-mediated unfolding by positioning Ufd1 for efficient substrate loading.

Findings

Accessory adapters FAF1, FAF2, and UBXN7 stimulate proteasomal degradation by increasing p97 unfolding rates.

A helix-UBX segment in FAF1/2 tethers Ufd1's ubiquitin-binding module to p97, improving substrate loading.

Disrupting the helix-Ufd1 interaction reduces degradation efficiency, confirming the role of accessory adapters.

Abstract

The AAA-ATPase VCP/p97 with its adapter Ufd1-Npl4 unfolds ubiquitylated substrate proteins to prepare degradation in the proteasome; however, the function of critical accessory factors remains unclear. Here, we show in the mammalian system that efficient protein degradation in the proteasome requires accessory adapters that boost p97-mediated unfolding likely by positioning Ufd1 for substrate loading. In a reaction that reconstitutes p97-Ufd1-Npl4–mediated unfolding coupled to proteasomal degradation, degradation was inefficient but stimulated by accessory adapters FAF1, FAF2, or UBXN7. Stimulation of proteasomal degradation was largely caused by an increase of p97 unfolding rates, conveyed by a helix-UBX segment in FAF1/2 that tethered the UT3 ubiquitin binding module of Ufd1 to the p97 N-domain. Mutations that abrogated the helix-Ufd1 interaction reduced stimulation of degradation, suggesting that accessory adapters position Ufd1 within the p97 complex to organize proficient substrate loading. Our results define the function of accessory adapters in mammals and highlight the complexity of substrate loading onto p97 for efficient substrate processing.

A group of accessory adapters positions the p97 adapter Ufd1 to boost substrate unfolding and subsequent proteasomal degradation.

Figures