MKLN1-dependent GID4/CTLH E3 ubiquitin ligase complex assemblies are required to support B-cell antibody diversification.

C-terminal to LisH (CTLH) E3 ubiquitin ligase complexes regulate a broad range of biological processes and forms separate supramolecular CTLH-MKLN1 and CTLH-WDR26 assemblies possessing distinct substrate specificities. Our previous work revealed that the CTLH complex utilizes the FAM72A substrate adaptor to ubiquitinate and degrade the uracil-DNA glycosylase 2 (UNG2) base excision repair factor. This outcome in B cells permits deoxyuridine mutations catalyzed by activation-induced cytidine deaminase (AID) to persist toward mutational outcomes and drive antibody diversification events. Here, we report that Mkln1-/- mice specifically lacking assembly of CTLH-MKLN1 complexes display reduced somatic hypermutation and class switch recombination frequencies due to increased UNG2, similar to Fam72a-/- mice. Strikingly, Mkln1-/- mice showed increased germinal center B cells and defects during B-cell development, a phenotype not observed in Fam72a-/- mice, suggesting that MKLN1 regulates proteins that are independent of FAM72A. Together, this work identifies that CTLH-MKLN1 ubiquitin E3 ligase complexes are critical in generating effective humoral immune responses and reveals distinctions between FAM72A-dependent and -independent CTLH complex modalities.