Plexin is a family of transmembrane protein conserved across a wide range of animal species. Recent studies on Drosophilae and mice showed that plexins act as receptors for semaphorins, a group of proteins originally identified as a chemorepulsive factor for growing axons. C. elegans has two genes for plexins,
plx-1 and
plx-2 , (pka.
cep-2 and
cep-1 , respectively); two genes for transmembrane semaphorins,
Ce-sema-1a ,
Ce-sema-1b ; and one gene for a secreted semaphorin,
mab-20/Ce-
sema-2a . We have generated deletion mutations for
plx-1 and
plx-2 in order to examine their function in vivo. Here we report the ray morphogenesis in these mutants.Putative null mutants of the
plx-1 gene exhibited anterior displacement of Ray1 in the adult male tail. The similar ray phenotype was observed by suppression of
Ce-sema-1a and/or
Ce-sema-1b by RNAi. In
plx-1 mutants, precursors for Ray1 were often mispositioned, suggesting that
plx-1 regulates the arrangement of the ray precursors. Showing no defects in other rays, the phenotype of
plx-1 mutants is distinct from that of
mab-20 mutants reported previously.
plx-2 (
nc7) is presumed to cause a deletion within the extracellular region of PLX-2. While
plx-2 (
nc7) animals exhibited no apparent defects in the male tail by itself,
nc7 enhanced the ray fusion traits of
mab-20(bx-24) , an allele of medium strength, suggesting that these genes interact. By using cultured cells, we also showed that Ce-Sema-1a binds to PLX-1, but not to PLX-2, while Ce-Sema-2a binds to PLX-2 but not to PLX-1. The results indicate that PLX-1 interacts with the transmembrane semaphorins and PLX-2 interacts with the secreted semaphorin. Although both plexins are required for the proper positioning of rays, they appear to function rather independently.