Functions as a calcium- and DAG-regulated nucleotide exchange factor specifically activating Rap through the exchange of bound GDP for GTP. May also activate other GTPases such as RRAS, RRAS2, NRAS, KRAS but not HRAS. Functions in aggregation of platelets and adhesion of T-lymphocytes and neutrophils probably through inside-out integrin activation. May function in the muscarinic acetylcholine receptor M1/CHRM1 signaling pathway. Forms a signaling complex with RAP1 and BRAF. Interacts with F-actin (By similarity). Interacts with RAP1. Found both in the cytosol and associated with membranes. Enriched at juxtamembrane areas and membrane ruffles. Localizes to the cell bodies and axons of striatal neurons. Detected in megakaryocytes, platelet and neutrophils but not in lymphocytes (at protein level). Isoform 1 and isoform 3 are detected in brain basal glanglia, heart, lung, spleen, liver and kidney interstitial cells. Expressed in embryo with higher expression between 15 dpc and 17 dpc. The N-terminal Ras-GEF domain mediates association with F-actin. Mice have a combination of defects in leukocytes and platelet functions which are reminiscent of the human leukocyte adhesion deficiency type III syndrome (LAD3). They display bleeding diathesis due to a defect in platelet aggregation and are resistant to collagen-induced thrombosis. In parallel, they also display impaired response to acute inflammation associated with defects in beta-1 and beta-2 integrin-mediated adhesion of neutrophils. The corresponding protein is not undetectable. Belongs to the RASGRP family.