Functions as a diacylglycerol (DAG)-regulated nucleotide exchange factor specifically activating Ras through the exchange of bound GDP for GTP. Regulated by F-actin polymerization and probably by calcium. Found both in the cytosol and associated with membranes. The phorbol-ester/DAG-type zinc finger is the principal mediator of the targeting to membranes and is required for functional activation through DAG-binding. Belongs to the RASGRP family.