SNS : A muscle specific cell adhesion protein essential for myoblast fusion. M. Chakravarti , B. Bour , J. West , M. Hazen , S. Abmayr. Department of Biochemistry and Molecular Biology, Pennsylvania State University, Universiy Park, PA 16802.

   Drosophila embryos carrying mutations in the sticks and stones locus exhibit a dramatic absence of muscle fibers and a large number of unfused myosin expressing myoblasts. By comparison, no obvious abnormalities have been observed in other tissues, suggesting a somatic muscle specific requirement for SNS. To determine its location, recombinational mapping experiments were performed that placed the sns locus at genetic position 58. The location of sns was further refined by localization between the proximal breakpoints of two overlapping deficiencies generated by imprecise excision of P elements in 44F. To clone the gene, a chromosomal walk was initiated using P1 clones that span the region of interest. A genomic fragment isolated from this walk revealed an 8 kb transcript on a Northern blot. Sequence lesions in this transcript have been detected in three EMS induced mutant alleles of sns, confirming that the cloned gene is responsible for the observed phenotype. Embryonic expression of the sns transcript begins around stage 11 in the somatic mesoderm just prior to fusion of myoblasts and persists strongly until stage 14 when fusion is essentially complete. The SNS protein is expressed in the same spatial and temporal manner as the transcript and appears to localize at the membrane of the fusing cells as myogenesis progresses. Primary expression of SNS is in the somatic musculature. However, faint expression is observed in muscle attachment sites and CNS during later stages, the significance of which is yet unclear. Further analysis revealed that SNS is preferentially expressed in the putative fusion competent myoblasts and not in the muscle founders. The complete cDNA sequence of sns encodes eight immunoglobulin domains, one fibronectin type III domain, a transmembrane domain and a cytoplasmic domain, suggesting that SNS is a cell adhesion molecule belonging to the immunoglobulin superfamily.