Functions as a cation- and diacylglycerol (DAG)-regulated nucleotide exchange factor activating Ras through the exchange of bound GDP for GTP. May function in mast cells differentiation. Recruited to membranes upon activation by DAG. Expressed by mast cells and their progenitors (at protein level). The phorbol-ester/DAG-type zinc finger mediates the binding and the functional activation by DAG. Belongs to the RASGRP family.