PCMT1 generates the C-terminal cyclic imide degron on CRBN substrates

The E3 ligase substrate adapter cereblon (CRBN), the primary target of clinical agents thalidomide and lenalidomide, recognizes endogenous substrates bearing the C-terminal cyclic imide modification. Although C-terminal cyclic imides can form spontaneously, an enzyme that regulates the formation of these modifications and thereby promotes a biological pathway connecting substrates to CRBN is unknown. Here, we report that protein carboxymethyltransferase (PCMT1) promotes formation of the C-terminal cyclic imide on C-terminal asparagine residues of CRBN substrates. PCMT1 and CRBN co-regulate the levels of metabolic enzymes glutamine synthetase (GLUL) and inorganic pyrophosphatase 1 (PPA1) in vitro, in cells, and in vivo, and this regulation is associated with the proepileptic phenotype of CRBN knockout mouse models. The discovery of an enzyme that regulates CRBN substrates through the C-terminal cyclic imide modification reveals a previously unknown biological pathway that is perturbed by thalidomide derivatives and provides a biochemical basis for the connection between multiple biological processes and CRBN.