FAINT SAUSAGE ENCODES A NOVEL EXTRACELLULAR PROTEIN OF THE IMMUNOGLOBULIN SUPERFAMILY REQUIRED FOR CELL-MIGRATION AND THE ESTABLISHMENT OF NORMAL AXONAL PATHWAYS IN THE DROSOPHILA NERVOUS-SYSTEM

We examined the structure of the nervous system in Drosophila embryos homozygous for a null mutation in the faint sausage (fas) gene. In the peripheral nervous system (PNS) offas mutants, neurons fail to delami nate from the ectodermal epithelium; in the central nervous system (CN S), the positions of neuronal cell bodies and glial cells are abnormal and normal axonal pathways do not form. Sequence analysis of fas cDNA s revealed that the fas protein product has characteristics of an extr acellular protein and that it is a novel member of the immunoglobulin (Ig) superfamily. In situ hybridization demonstrated that fas transcri pts are expressed throughout the embryo but they are in relatively hig h concentrations in the lateral ectoderm, from which the peripheral ne rvous system delaminates and in the CNS. Antiserum directed against Fa s protein was found to stain neurons but not glia in the CNS, We concl ude that fas encodes a protein that, in the developing nervous system, is present on the surface of neurons and is essential for nerve cell migration and the establishment of axonal pathways.