A major type of spontaneous protein damage that accumulates with age is the formation of kinked polypeptide chains with L-isoaspartyl residues. Mitigating this damage is necessary for maintaining proteome stability and prolonging organismal survival. While repair through methylation by PCMT1 has been previously shown to suppress L-isoaspartyl accumulation, we provide an additional mechanism for L-isoaspartyl maintenance through PCMTD1, a cullin-RING ligase (CRL). We combined cryo-EM, native mass spectrometry, and biochemical assays to provide insight on how the assembly and architecture of human PCMTD1 in the context of a CRL complex fulfils this alternative mechanism. We show that the PCMTD1 CRL complex specifically binds L-isoaspartyl residues when bound to AdoMet. This work provides evidence for a growing class of E3 ubiquitin ligases that recognize spontaneous covalent modifications as potential substrates for ubiquitylation and subsequent proteasomal degradation.