Asymmetric cell divisions in the developing C.elegans nervous system generate both apoptotic cells and differentiated neurons. The mechanisms that specify distinct apoptotic versus neural fates in these dividing cells are largely unknown. We have identified the cytohesin GRP-1, a Guanine Nucleotide Exchange Factor (GEF) for the ARF family of small GTPases; two GTPases, ARF-1 and ARF-6; and the ARF GTPase Activating Protein (GAP) CNT-2 as regulators of the asymmetric division of the Q.p neuroblast. Loss of these molecules results in the survival of Q.pp, a cell normally fated to die, as well as its transformation into the A/PVM-SDQ precursor, its sister cell, resulting in extra A/PVM and SDQ neurons. Homologs of these molecules have been shown to regulate membrane trafficking and cell signaling. Our structure-function analyses show that the GEF activity of GRP-1 is both necessary and sufficient for its function and that the GAP domain of CNT-2 is necessary for its function. Expression of GRP-1 from specific promoters suggests that it functions cell non-autonomously and thus may regulate a signal that controls the Q.p division. GRP-1 localizes to the midbody, raising the interesting possibility that this structure might be involved in cell signaling. Based on several criteria, we come to the surprising conclusion that GRP-1 and CNT-2 define two independent ARF GTPase cycles that regulate asymmetric neuroblast divisions. First, while
grp-1 mutations do not alter the sizes of the Q.p daughter cells,
cnt-2 mutations do. Second, GRP-1 and CNT-2 have distinct sub-cellular localization patterns. Third, genetic interactions suggest that ARF-1 functions in parallel to GRP-1. Finally, the most compelling evidence indicating that GRP-1 and CNT-2 play different roles in the Q.p division comes from the finding that CNT-2 acts autonomously in the Q.p division, revealing that GRP-1 and CNT-2 act in different cells. Taken together, our studies have for the first time shown in an in vivo context that ARF-mediated pathways regulate asymmetric cell divisions, and that they do so both cell autonomously as well as by regulating a signal.