Key regulator of the integrin-mediated cell-matrix interaction signaling by binding to the ITGB1 cytoplasmic tail and preventing the activation of integrin alpha-5/beta-1 (heterodimer of ITGA5 and ITGB1) by talin or FERMT1. Plays a role in cell proliferation, differentiation, spreading, adhesion and migration in the context of mineralization and bone development and angiogenesis. Stimulates cellular proliferation in a fibronectin-dependent manner. Involved in the regulation of beta-1 integrin-containing focal adhesion (FA) site dynamics by controlling its assembly rate during cell adhesion; inhibits beta-1 integrin clustering within FA by directly competing with talin TLN1, and hence stimulates osteoblast spreading and migration in a fibronectin- and/or collagen-dependent manner. Acts as a guanine nucleotide dissociation inhibitor (GDI) by regulating Rho family GTPases during integrin-mediated cell matrix adhesion; reduces the level of active GTP-bound form of both CDC42 and RAC1 GTPases upon cell adhesion to fibronectin. Stimulates the release of active CDC42 from the membranes to maintain it in an inactive cytoplasmic pool. Participates in the translocation of the Rho-associated protein kinase ROCK1 to membrane ruffles at cell leading edges of the cell membrane, leading to an increase of myoblast cell migration on laminin. Plays a role in bone mineralization at a late stage of osteoblast differentiation; modulates the dynamic formation of focal adhesions into fibrillar adhesions, which are adhesive structures responsible for fibronectin deposition and fibrillogenesis. Plays a role in blood vessel development; acts as a negative regulator of angiogenesis by attenuating endothelial cell proliferation and migration, lumen formation and sprouting angiogenesis by promoting AKT phosphorylation and inhibiting ERK1/2 phosphorylation through activation of the Notch signaling pathway. Promotes transcriptional activity of the MYC promoter. Found in a complex, at least composed of ITGB1BP1, KRIT1 and RAP1A. Interacts (via C-terminal region) with ITGB1 (via C-terminal cytoplasmic tail); the interaction prevents talin TLN1 binding to ITGB1 and KRIT1 and ITGB1 compete for the same binding site. Interacts with KRIT1 (via N-terminal NPXY motif); the interaction induces the opening conformation of KRIT1 and KRIT1 and ITGB1 compete for the same binding site. Isoform 2 does not interact with ITGB1. Interacts with CDC42 (GTP- or GDP-bound form); the interaction is increased with the CDC42-membrane bound forms and prevents both CDC42 activation and cell spreading. Interacts (via C-terminal domain region) with NME2. Interacts with FERMT2 and RAC1 (By similarity). Interacts (via N-terminus and PTB domain) with ROCK1. Nucleocytoplasmic shuttling protein; shuttles between nucleus and cytoplasm in an integrin-dependent manner; probably sequestered in the cytosol by ITGB1. Its localization is dependent on the stage of cell spreading on fibronectin; cytoplasmic in case of round cells, corresponding to the initial step of cell spreading, or nuclear in case of well spread cells. Colocalizes with ROCK1 and NME2 at beta-1 integrin engagement sites. Together with ITGB1 and NME2 is recruited to beta-1 integrin-rich peripheral ruffles and lamellipodia during initial cell spreading on fibronectin and/or collagen (By similarity). Expressed in the brain. First expressed in the heart primordium at 8.5 dpc. Phosphorylation at Thr-38 seems to enhance integrin alpha5beta1-mediated cell adhesion. The degree of phosphorylation is regulated by integrin-dependent cell-matrix interaction (By similarity). Mice show severe defects in osteoblast mesenchymal cells to compaction, proliferation, differentiation, and mineralization and to a delay in bone nodule formation. Suffer also from an excessive microvessel growth.