In an attempt to identify proteins that regulating synapse formation, we previously described a screen for mutations that alter GFP-tagged synaptic vesicle localization in mechanosensory neurons. Mutants from this screen, designated sam (synaptic vesicle tag abnormal in mechanosensory neurons), were classified based on several phenotypic criteria. These criteria suggest that the genes (
rpm-1 V,
sam-3 II,
sam-6 II, and
sam-8 X) likely regulate aspects of synapse development in multiple distinct cell types.
rpm-1(aka
sam-1) encodes an extremely large protein expressed throughout the nervous system. The primary sequence contains motifs suggesting protein-protein interactions and exchange factor activity. In C. elegans, RPM-1 is required during synapse formation, and acts cell-autonomously in presynaptic mechanosensory neurons. RPM-1 is conserved, with homologs in Drosophila and human. Biochemical functions of these homologous proteins have not yet been described, though mutations in the Drosophila homolog also expressed a synaptogenesis defect.
sam-6 and
sam-8 exhibit very similar phenotypes as
rpm-1. Each exhibits a variably penetrant defect in formation of GFP-tagged synaptic varicosities. Double mutant combinations between each of the three loci resemble
rpm-1 mutants in severity. Progress towards molecular cloning of
sam-6 and
sam-8 will be described.
sam-3 shows a similar defect in accumulation of SNB-GFP in the ventral nerve cord, but in addition, exhibits an accumulation of SNB-1-GFP and altered morphology of soma. In
rpm-1 sam-3 double mutants the
sam-3 cell body phenotype is suppressed sugggesting that
rpm-1 is required for expression of
sam-3 defects. Furthermore,
sam-3 exhibits several other phenotypes seen in
rpm-1 mutants. SAB neurons exhibit altered morphology, and the lateral process of PLM axons often over grow and desend into the ventral nerve cord. Our current working hypothesis is that
sam-3 regulates
rpm-1 activity or localization.
sam-3 has been mapped to a 0.2 map unit interval of II.