A NOVEL TRANSMEMBRANE SEMAPHORIN CAN BIND C-SRE

The semaphorins/collapsins constitute a family of genes unified by the presence of a ''semaphorin domain'' which has been conserved through metazoan evolution. The semaphorin family comprises both secreted and transmembrane molecules and is thought to be made up of ligands for as yet unidentified receptors. The functions are not known, with the exc eption of those of sema III (also referred as sem D and collapsin 1), D-sema I, and D-sema II, which have been shown to be involved in axona l pathfinding. Here we report the identification of a mouse semaphorin cDNA, termed Sema VIb. Although Sema VIb contains the extracellular s emaphorin domain, it lacks the immunoglobulin domain or thrombospondin repeats which are present in other described vertebrate (but not inve rtebrate) transmembrane semaphorins. During development Sema VIb mRNA is expressed in subregions of the nervous system and is particularly p rominent in muscle. In adulthood, Sema VIb mRNA is expressed ubiquitou sly. The cytoplasmic domain of Sema VIb contains several proline-rich potential SH3 domain binding sites. Using an in vitro binding assay, w e show that Sema VIb binds specifically the SH3 domain of the protoonc ogene c-src. In transfected COS cells Sema VIb coimmunoprecipitates wi th c-src. These results, along with our evidence that Sema VIb can for m dimers, suggests that the semaphorin family not only serves as ligan ds but may include members, especially those which are transmembrane, which serve as receptors, triggering intracellular signaling via an sr c-related cascade.