Novel signaling pathways regulating KRIT1 nucleocytoplasmic shuttling and its role in CCM pathogenesis.

KRIT1 is a disease gene responsible for Cerebral Cavernous Malformations (CCM), a human disease characterized by abnormally enlarged and leaky capillaries that predispose to seizures, focal neurological deficits, and fatal intracerebral hemorrhage. It encodes for a protein containing distinct protein-protein interaction domains, including three NPXY/F motifs and a FERM domain. Previously, we isolated KRIT1B, an isoform characterized by the alternative splicing of the 15th coding exon and suspected to cause CCM when abnormally expressed. We found that KRIT1B is devoid of the F3/PTB-like subdomain of the FERM domain and is unable to bind Rap1A, while the FERM domain of KRIT1A is even sufficient for this function. Furthermore, a functional PTB subdomain enables the nucleocytoplasmic shuttling of KRIT1A, while its alteration confers a restricted cytoplasmic localization and a dominant negative role to KRIT1B. Importantly, we also defined a mechanism whereby an open/closed conformation switch regulates KRIT1A nuclear translocation and interaction with Rap1A in a mutually exclusive manner. Finally, we identified a novel signaling pathway that regulate the nucleocytoplasmic shuttling of KRIT1A as well as a novel way in which KRIT1 loss-of-function mutations may contribute to CCM pathogenesis.