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Fig. 7. Working model of GPA-16 activation during asymmetric cell division. RIC-8 binds to GPA-16·GDP, counteracting the formation of the GPA-16/Gß{gamma} heterotrimer; this allows association of GPR-1/2 and LIN-5 with GPA-16. A complex of GPA-16·GDP-GPR1/2-LIN-5 promotes generation of pulling forces on spindle poles. As some GPR-1/2 co-immunoprecipitates with RIC-8 (Afshar et al., 2004), RIC-8 may still be present in this complex, as illustrated. As RIC-8 does not exhibit GEF activity towards GPA-16, RIC-8-independent nucleotide exchange may promote formation of GPA-16·GTP. Co-immunoprecipitation experiments (see Fig. 5G) suggest that RIC-8 can associate with GPA-16·GTP, although this is not illustrated. RGS-7 GAP activity promotes GTP hydrolysis and formation of GPA-16·GDP and is thus is required for the activation cycle (Hess et al., 2004).