During epithelial cell polarization, aPKC phosphorylates Yurt to prevent its premature apical localization, while at the same time Yurt binds to and restrains aPKC function.

Yurt (Yrt) and its mammalian orthologue EPB41L5 limit apical membrane growth in polarized epithelia. EPB41L5 also supports epithelial-mesenchymal transition and metastasis. Yrt and EPB41L5 contain a four-point-one, ezrin, radixin, and moesin (FERM) domain and a FERM-adjacent (FA) domain. The former contributes to the quaternary structure of 50 human proteins, whereas the latter defines a subfamily of 14 human FERM proteins and fulfills unknown roles. In this study, we show that both Yrt and EPB41L5 oligomerize. Our data also establish that the FERM-FA unit forms an oligomeric interface and that multimerization of Yrt is crucial for its function in epithelial cell polarity regulation. Finally, we demonstrate that aPKC destabilizes the Yrt oligomer to repress its functions, thereby revealing a mechanism through which this kinase supports apical domain formation. Overall, our study highlights a conserved biochemical property of fly and human Yrt proteins, describes a novel function of the FA domain, and further characterizes the molecular mechanisms sustaining epithelial cell polarity.

Fanconi Anemia (FA) clinical phenotypes are heterogenous and rely on a mutation in one of the 22 FANC genes (FANCA-W) involved in a common interstrand DNA crosslink-repair pathway. A critical step in the activation of FA pathway is the monoubiquitination of FANCD2 and its binding partner FANCI. To better address the clinical phenotype associated with FANCI and the epistatic relationship with FANCD2, we created the first conditional inactivation model for FANCI in mouse. Fanci −/− mice displayed typical FA features such as delayed development in utero, microphtalmia, cellular sensitivity to mitomycin C, occasional limb abnormalities and hematological deficiencies. Interestingly, the deletion of Fanci leads to a strong meiotic phenotype and severe hypogonadism. FANCI was localized in spermatocytes and spermatids and in the nucleus of oocytes. Both FANCI and FANCD2 proteins co-localized with RPA along meiotic chromosomes, albeit at different levels. Consistent with a role in meiotic recombination, FANCI interacted with RAD51 and stimulated D-loop formation, unlike FANCD2. The double knockout Fanci−/− Fancd2−/− also showed epistatic relationship for hematological defects while being not epistatic with respect to generating viable mice in crosses of double heterozygotes. Collectively, this study highlights common and distinct functions of FANCI and FANCD2 during mouse development, meiotic recombination and hematopoiesis.