Component of the CCM signaling pathway which is a crucial regulator of heart and vessel formation and integrity. Negative regulator of angiogenesis. Inhibits endothelial proliferation, apoptosis, migration, lumen formation and sprouting angiogenesis in primary endothelial cells. Promotes AKT phosphorylation in a NOTCH-dependent and independent manner, and inhibits ERK1/2 phosphorylation indirectly through activation of the DELTA-NOTCH cascade. Acts in concert with CDH5 to establish and maintain correct endothelial cell polarity and vascular lumen and these effects are mediated by recruitment and activation of the Par polarity complex and RAP1B. Required for the localization of phosphorylated PRKCZ, PARD3, TIAM1 and RAP1B to the cell junction, and cell junction stabilization. Plays a role in integrin signaling via its interaction with ITGB1BP1; this prevents the interaction between ITGB1 and ITGB1BP1. Microtubule-associated protein that binds to phosphatidylinositol 4,5-bisphosphate (PIP2)-containing membranes in a GTP-bound RAP1-dependent manner (By similarity). Plays an important role in the maintenance of the intracellular reactive oxygen species (ROS) homeostasis to prevent oxidative cellular damage. Regulates the homeostasis of intracellular ROS through an antioxidant pathway involving FOXO1 and SOD2. Facilitates the down-regulation of cyclin-D1 (CCND1) levels required for cell transition from proliferative growth to quiescence by preventing the accumulation of intracellular ROS through the modulation of FOXO1 and SOD2 levels. May play a role in the regulation of macroautophagy through the down-regulation of the mTOR pathway (PubMed:26417067). Found in a complex, at least composed of ITGB1BP1, KRIT1 and RAP1A. Interacts (via C-terminus FERM domain) with RAP1A (active GTP-bound form preferentially); the interaction does not induce the opening conformation of KRIT1. Interacts (via N-terminus NPXY motif) with ITGB1BP1; the interaction induces the opening conformation of KRIT1 and competes with ITGB1 for ITGB1BP1 interaction. Associates (via N-terminus and C-terminus regions) with microtubules; the interaction is inhibited in presence of ITGB1BP1 and active GTP-bound RAP1A. Interacts (via FERM domain) with RAP1B. Interacts with CDH5. Interacts with RAP1A (By similarity). Interacts with HEG1 and CCM2; greatly facilitates CCM2-binding to HEG1 (PubMed:19151727). KRIT1 and CDH5 reciprocally regulate their localization to endothelial cell-cell junctions. Association with RAP1 relocalizes KRIT1 from microtubules to cell junction membranes. Translocates from the cytoplasm along microtubules to the cell membrane in an ITGB1BP1-dependent manner (By similarity). Expressed in heart, brain, spleen, lung, thymus, kidney and testis. Isoform 2 was more frequently expressed in the thymus than isoform 1. At stage 9.5 dpc ubiquitously expressed, at 12.5 dpc expressed in structures of the CNS, especially in zones of the proliferative active ventricular zones of the brain and in the spinal cord. Expression increased in organs that were in the state of organ expansion like lung and liver. At 17.5 dpc, expression was strongly reduced in endoderm-derived tissues. In early postnatal development, strongly expressed in regions of ossification. The FERM domain mediates binding to RAP1A and RAP1B and is necessary for binding to phosphatidylinositol 4,5-bisphosphate (PIP2). The N-terminal domain has structural similarity to the nudix hydrolase domain, despite the absence of a nudix box and low sequence similarity with nudix hydrolase domains. The N-terminus and the C-terminus part associate together via the NPAY binding motif and adopt a lose conformation that is disrupted by ITGB1BP1, but not by RAP1A. Contains 4 ANK repeats that precede the FERM domain.