The BCR gene is one of the two genes in the BCR-ABL complex, which is associated with the Philadelphia chromosome, a product of a reciprocal translocation between chromosomes 22 and 9. BCR is a GTPase-activating protein (GAP) for RAC1 (primarily) and CDC42. The Dbl region of BCR has the most RhoGEF activity for Cdc42, and less activity towards Rac and Rho. Since BCR possesses both GAP and GEF activities, it may function to temporally regulate the activity of these GTPases. It also displays serine/threonine kinase activity. The BCR protein contains multiple domains including an N-terminal kinase domain, a RhoGEF domain, a PH domain, a C1 domain, a C2 domain, and a C-terminal RhoGAP domain. ABR, a related smaller protein, is structurally similar to BCR, but lacks the N-terminal kinase domain and has GAP activity for both Rac and Cdc42. PH domains have diverse functions, but in general are involved in targeting proteins to the appropriate cellular location or in the interaction with a binding partner. They share little sequence conservation, but all have a common fold, which is electrostatically polarized. Less than 10% of PH domains bind phosphoinositide phosphates (PIPs) with high affinity and specificity. PH domains are distinguished from other PIP-binding domains by their specific high-affinity binding to PIPs with two vicinal phosphate groups: PtdIns(3,4)P2, PtdIns(4,5)P2 or PtdIns(3,4,5)P3 which results in targeting some PH domain proteins to the plasma membrane. A few display strong specificity in lipid binding. Any specificity is usually determined by loop regions or insertions in the N-terminus of the domain, which are not conserved across all PH domains. PH domains are found in cellular signaling proteins such as serine/threonine kinase, tyrosine kinases, regulators of G-proteins, endocytotic GTPases, adaptors, as well as cytoskeletal associated molecules and in lipid associated enzymes.
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