Component of the CCM signaling pathway which is a crucial regulator of heart and vessel formation and integrity. May act through the stabilization of endothelial cell junctions. May also function as a scaffold protein for MAP2K3-MAP3K3 signaling. Seems to play a major role in the modulation of MAP3K3-dependent p38 activation induced by hyperosmotic shock (By similarity). Part of a complex with MAP2K3, MAP3K3 and RAC1. Binds RAC1 directly and independently of its nucleotide-bound state (By similarity). Interacts with HEG1 and KRIT1; KRIT1 greatly facilitates the interaction with HEG1. Expressed in the ventricular zone of the brain at 28 hours post fertilization (hpf) and, at lower levels, in the posterior cardinal vein and in the posterior intermediate inner cell mass. At 48 hpf, still detected in the vein. At 28 and 48 hpf, expressed at low levels in a region near the dorsal aorta. The C-terminal region constitutes an independently folded domain that has structural similarity with the USH1C (harmonin) N-terminus, despite very low sequence similarity. The heart chambers in mutant animals are huge, constituted of a monolayered myocardium lined by endocardium. Belongs to the CCM2 family.