In wild type L1 animals DD motorneurons form neuromuscular junctions (NMJs) with ventral body wall muscles. Following the birth of VD motorneurons in the late L1 stage, DDs undergo synaptic remodeling: their ventral NMJs are eliminated while new NMJs are formed on the dorsal side (J. White et al., (1978) Nature 271, 764-766). The nascent VDs form NMJs with ventral body wall muscles. We isolated the
ju2 mutation in a screen for abnormal synaptic patterns of the type D neurons using the [Punc-25VAMP-GFP] array as a marker (see abstract by M. Zhen and Y. Jin this gazette). This array carries a fusion gene of the green fluorescent protein (GFP) and the C. elegans vesicle associated membrane protein (VAMP) that is driven by the
unc-25 promoter. The GFP is specifically localized to the presynaptic zones of DDs and VDs (Y. Jin and B. Horvitz, 1995 International Worm Meeting abstract 291). In
ju2 mutant animals the synaptic specificity of type D neurons is changed without alterations in cell morphology. The
ju2 mutation causes premature remodeling of juvenile DDs such that DDs form NMJs onto the dorsal body wall muscles in early L1 animals. In older larvae and adults the
ju2 mutation causes a reduction in the number of ventral NMJs made from VDs.
ju2 mutant animals show a ventral coiler phenotype when backing and also exhibit mild defects in egg laying. Based on the mutant phenotype, we have named the gene
syd-1, for synapse defective. We mapped
syd-1(
ju2) to the cluster of linkage group II. Animals hemizygous for
ju2 over a deficiency exhibit a similar ventral coiler phenotype to
ju2 homozygotes, suggesting that
ju2 is likely a null allele for
syd-1. We obtained germline transformation rescue of
syd-1 with a 15.6 kb minimal rescuing sequence. The minimal rescuing sequence carries onepredicted gene with sequence homology to GTPase activating proteins (GAPs) such as N-Chimaerin and the gene product of the break point cluster region (BCR). We have introduced a stop codon in the predicted gene, injection of which failed to rescue the
ju2 mutant phenotype. The GAP domain of the predicted SYD-1 bears 28% sequence identity and 51% sequence similarity to that of BCR. The GAP domain of BCR functions as a regulator of the small GTPases Rac and Cdc42Hs. Both Rac and Cdc42Hs have been implicated in the clustering of integrins and the formation of focal complexes (Nobes, C.D. and Hall A. (1995) Cell 81, 53-63). The sequence similarity between
syd-1 and BCR hints at the possibility that
syd-1 may regulate the organization of the actin cytoskeleton of type D motorneurons. On Northerns we detected a 3.4 kb transcript for
syd-1. We have obtained a partial cDNA corresponding to this transcript (from Y. Kohara) and are in the process of obtaining the complete 5 prime portion of the message. In addition, we have initiated the construction of GFP reporters with which to determine the expression pattern of
syd-1. Previously we showed that the temporal regulation of DD synaptic remodeling is under the control of the heterochronic genes
lin-4 and
lin-14 (S. Hallam and Y. Jin 1996 West Coast Worm Meeting abstract 108). We have begun to investigate potential genetic interactions between
syd-1 and
lin-4 and
lin-14.