C-terminal amides mark proteins for degradation via SCF/FBXO31

Abstract During normal cellular homeostasis unfolded and mis-localized proteins are recognized and removed, preventing the build-up of toxic byproducts 1 . When protein homeostasis is perturbed during aging, neurodegeneration or cellular stress, proteins can accumulate several forms of chemical damage through reactive metabolites 2, 3 . Such modifications have been proposed to trigger the selective removal of chemically marked proteins 3–6; however, discovering modifications sufficient to induce protein degradation has remained challenging. Using a semi-synthetic chemical biology approach coupled to cellular assays, we found that C-terminal amide-bearing proteins (CTAPs) are rapidly cleared from human cells. A CRISPR screen identified the SCF/FBXO31 ubiquitin ligase as a reader of C-terminal amides, which ubiquitylates CTAPs for subsequent proteasomal degradation. A conserved binding pocket enables FBXO31 to bind almost any C-terminal peptide bearing an amide while retaining exquisite selectivity over non-modified clients. This mechanism facilitates binding and turnover of endogenous CTAPs that are formed following oxidative stress. A dominant human mutation found in neurodevelopmental disorders switches CTAP recognition, such that non-amidated neosubstrates are now degraded and FBXO31 becomes markedly toxic. We propose that CTAPs may represent the vanguard of a largely unexplored class of modified amino acid degrons that could provide a general strategy for selective yet broad surveillance of chemically damaged proteins.