THE SEMAPHORIN GENES ENCODE A FAMILY OF TRANSMEMBRANE AND SECRETED GROWTH CONE GUIDANCE MOLECULES

In addition to its expression on subsets of axons, grasshopper Semapho rin I (Sema I, previously called Fasciclin [Fas] IV) is expressed on a n epithelial stripe in the limb bud, where it functions in the guidanc e of two sensory growth cones as they abruptly turn upon encountering this sema I boundary. We report here on the cloning and characterizati on of two sema genes in Drosophila, one in human, and the identificati on of two related viral sequences, all of which encode proteins with c onserved Semaphorin domains. Drosophila, sema (D-Sema) I is a transmem brane protein, while D-Sema II and human Sema III are putative secrete d proteins that are similar to the recently reported chick collapsin. D-Sema I and D-Sema II are expressed by subsets of neurons and muscles . Genetic analysis in Drosophila reveals that semaII is an essential g ene that is required for both proper adult behavior and survival.