Rab GTPases are essential for vesicular transport. Rab GDP dissociation inhibitor (GDI) binds to GDP-bound rabs, removes rabs from acceptor membranes and delivers rabs to donor membranes. We isolated lethal GDI mutations in Drosophila and analyzed their developmental phenotypes. To learn how these mutations affect GDI structure, the crystal structure of Drosophila GDI was determined by molecular replacement to a resolution of 3.0 A. Two hypomorphic, missense mutations are located in domain II of GDI at highly conserved positions, but not in previously identified sequence conserved regions. The mutant GDIs were tested for ability to extract rabs from membranes and showed wild-type levels of rab membrane extraction. The two missense alleles showed intragenic complementation, indicating that domain II of GDI may have two separable functions. This study indicates that GDI function is essential for development of a complex, multicellular organism and that puparium formation and pole cell formation are especially dependent on GDI function.

Ras-mediated signaling is required for induction of vulval cell fates during Caenorhabditis elegans development. By screening for suppressors of the multivulva phenotype caused by constitutively active let-60 ras, we identified the mutation n2527. To clone the gene affected by n2527, we developed a method for high-resolution mapping. We took advantage of the genomic DNA sequence of the N2 strain by using DNA sequencing to scan for single-nucleotide polymorphisms (SNPs) at defined genomic positions of the RC301 strain. An average of one polymorphism per 1.4 kb was detected in predicted intergenic regions. Because of this high frequency, DNA sequencing is an efficient method to scan for SNPs. By alternating between identifying SNPs and mapping n2527 using selected recombinants, we generated an SNP map of progressively higher density. An intensive search for SNPs resulted in a local map with an average marker spacing of ∼4 kb. This was used to map n2527 to a 9.6-kb interval. The small size of this interval made it feasible to use DNA sequencing to identify the molecular lesion. In principle, this approach can be used for high-resolution mapping of any C. elegans mutation. Furthermore, this approach can be applied to other species as the genomic sequence becomes available. The n2527 mutation affects a previously uncharacterized gene that we named cdf-1, as it encodes a predicted protein with significant similarity to members of the cation diffusion facilitator family.