Abstract
Little is known about the signaling pathways by which motoneurons induce synapses on muscle fibers, and no receptors for synapse-inducing signals have yet been identified. Because several other inductive events in development are mediated by receptor tyrosine kinases (RTKs), and because phosphotyrosine staining within muscle fibers is concentrated at synaptic sites, one possibility is that synapse-inducing signals are transduced by a RTK within the muscle fiber. We have used PCR to search for tyrosine kinases within the electric organ of the electric ray Torpedo californica, since this tissue is homologous to muscle but is much more densely innervated and is therefore a rich source of synaptic molecules. We have isolated a RTK that is specifically expressed in electric organ and skeletal muscle. The kinase domain of this receptor is related to the trk family of neurotrophin receptors, but unlike any previously described receptor, the extracellular region of this Torpedo RTK contains a kringle domain close to the transmembrane domain.