The let-60 ras gene acts in a signal transduction pathway to control vulval differentiation in Caenorhabditis elegans. By screening suppressors of a dominant negative let-60 ras allele, we isolated three loss-of-function mutations in the sur-5 gene which appear to act as negative regulators of let-60 ras during vulval induction. sur-5 mutations do not cause an obvious mutant phenotype of their own, and they appear to specifically suppress only one of the two groups of let-60 ras dominant negative mutations, suggesting that the gene may be involved in a specific aspect of Ras activation. Consistent with its negative function, overexpressing sur-5 from an extragenic array partially suppresses the Multivulva phenotype of an activated let-60 ras mutation and causes synergistic phenotypes with a lin-45 raf mutation. We have cloned sur-5 and shown that it encodes a novel protein. We have also identified a potential mammalian SUR-5 homolog that is about 35% identical to the worm protein. SUR-5 also has some sequence similarity to acetyl coenzyme A synthetases and is predicted to contain ATP/GTP and AMP binding sites. Our results suggest that sur-5 gene function may be conserved through evolution.The Ras-mediated signal transduction pathway plays important roles in specifying cell fates in a number of developmental events in the nematode Caenorhabditis elegans including vulval cell differentiation (17, 31), male spicule cell differentiation (4), germ nucleus exit from pachytene (5), sex-myoblast migration (32), and excretory duct cell differentiation (38). Vulval differentiation has been the main system used in a number of laboratories to identify new components of this pathway and study its regulation. Vulval differentiation in C. elegans hermaphrodites is controlled by the combination of several cellcell signaling events ( Fig. 1A) (17,31). In particular, an inductive signal from the anchor cell induces three of the six vulval precursor cells (VPCs, P3.p to P.8p) to differentiate into vulval cells (Fig. 1A). The let-60 ras gene acts in a conserved signal transduction cascade to transduce the anchor cell signal encoded by the lin-3 gene (Fig. 1C). Previous genetic screens in several laboratories have identified many components that are either key factors acting in the main backbone of the signaling cascade (Fig. 1C) or factors that regulate the activity of these key players in the pathway (17, 31). The functions of several genes as regulators of the signaling pathway (e.g., unc-101, sli-1, and ksr-1) have been identified only by the genetic suppressor phenotypes and the synergistic effects of their mutations in other mutant backgrounds, since mutants with loss-offunction mutations in these genes show few or no abnormalities in vulval development (14,18,19,30).To identify negative factors that down regulate the let-60 ras activity, we screened for mutations that suppress the Vulvaless phenotype caused by a let-60 ras dominant negative (dn) mutation, let-60(K16N). In this paper, we describe the gene sur-5, defined by thr...
A fusion of the sur-5 protein to the green fluorescent protein containing a nuclear localization signal is demonstrated as a marker for genetic mosaic analysis in the nematode Caenorhabditis elegans. Because of an extensive accumulation of bright fluorescence in many nuclei, normal growth plates, each containing hundreds of worms, can be rapidly screened with a dissecting microscope for rare mosaic individuals. As the marker can also be used to detect transgenic worms, the construction of strains for mosaic analyses can be minimized. In the course of examining rare mosaic animals, an unexpected pattern of fluorescence was noticed for hyp6, a syncytial component of the hypodermis, which indicated that the marker may serve as a means of assessing cellular fusions during development. Immunofluorescent staining of adherens junctions confirmed a postembryonic fusion of hyp6 with hyp7, the major syncytium of the hypodermis.
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