UC San Diego, Department of Bioengineering

Rap1 GTPases regulate cell-matrix and cell-cell adhesion by interacting with specific Rap effectors. Cell-directed increases in Integrin affinity (“activation”) controls cell migration, matrix assembly, leukocyte trafficking, and hemostasis. Talin binding to the integrin β cytoplasmic domain is a final step in activation. Rap1 induces the activation of integrins by binding to RIAM, the product of the APBB1IP gene, to promote talin-dependent integrin activation. We find that that RIAM connects the membrane targeting sequences in Ras GTPases to talin, thereby recruiting talin to the plasma membrane and activating integrins. To prove this, we created a minimized 50 residue Rap-RIAM module containing the talin binding site of RIAM joined to the membrane-targeting sequence of Rap1A that targeted talin to the plasma membrane and mediated integrin activation. KRIT1 is a Rap effector and genetic deletions of KRIT1, CCM2, or CCM3 in humans and other vertebrates produce similar cardiovascular phenotypes. KRIT1 functions as a scaffold by physically interacting with CCM2 and thus CCM3 participates in multiprotein complexes containing Rho GTPases and numerous kinases and phosphatases. The genetic and biochemical relationships of KRIT1 and CCM2-3 indicate that the assembly and regulation of these multiprotein complexes is an important regulator of cardiovascular development. We found that KRIT1 localizes to endothelial and epithelial cell junctions and controls their integrity. Rap1 regulated the junctional localization of KRIT1, which is mediated by the interaction of KRIT1 with the transmembrane receptor, heart of glass (Heg1) and previous work showed that deletion of Heg1 produced a similar phenotype to loss of KRIT1 in zebrafish. These KRIT1 multiprotein complexes act through Rho GTPases to control the cytoskeleton and thus, junctional integrity.