THE MESODERMAL EXPRESSION OF ROLLING STONE (ROST) IS ESSENTIAL FOR MYOBLAST FUSION IN DROSOPHILA AND ENCODES A POTENTIAL TRANSMEMBRANE PROTEIN

In homozygous rolling stone embryos, the fusion of myoblasts to syncyt ial myotubes is diminished. Nevertheless, the visceral mesoderm, the h eart mesoderm, and few somatic muscles are properly formed, Thus, we p ostulate a central role of rolling stone for the fusion process within the somatic mesoderm. We have cloned the rolling stone gene, and the deduced protein sequence is in accordance with a transmembrane protein , which agrees with the enrichment of Rost in the membrane fraction of Drosophila embryos, No homologous genes have been described so far. r olling stone is expressed in the embryonic nervous system and cells of the somatic mesoderm, most notable in muscle founder cells. To elucid ate the function of rolling stone for myoblast fusion, we applied a kn ock-out strategy. The expression of an antisense rolling stone transcr ipt specifically within the mesoderm of wild-type embryos results in f usion defects of myoblasts, proving that the rolling stone expression in the mesoderm is responsible for the rolling stone phenotype. We sug gest that rolling stone is a member of a group of genes that are neces sary for the fusion process during myogenesis.