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

During normal cellular homeostasis unfolded and mis-localized proteins are recognized and removed, preventing the build-up of toxic byproducts1. When protein homeostasis is perturbed during aging, neurodegeneration or cellular stress, proteins can accumulate several forms of chemical damage through reactive metabolites2, 3. Such modifications have been proposed to trigger the selective removal of chemically marked proteins3-6; 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.