Functions as guanine nucleotide exchange factor (GEF) for Rab small GTPases. Activates specifically RABF1, RABF2A and RABF2B proteins. Required for early stages of embryogenesis, cytokinesis, embryogenesis, and organ development. Is essential for the establishment or maintenance of the polar localization of the auxin efflux carrier PIN1. Homodimer. The homodimer interacts with RABF2B. Interacts with RABF1 and RABF2A. A number of isoforms are produced. According to EST sequences. Widely expressed. Embryonic lethality when homozygous. Embryogenesis arrested at the torpedo stage.