NEURONAL AND NONNEURONAL COLLAPSIN-1 BINDING-SITES IN DEVELOPING CHICK ARE DISTINCT FROM OTHER SEMAPHORIN BINDING-SITES

The collapsin and semaphorin family of extracellular proteins contribu tes to axonal path finding by repulsing axons and collapsing growth co nes. To explore the mechanism of collapsin-1 action, we expressed and purified a truncated collapsin-1-alkaline phosphatase fusion protein ( CAP-4). This protein retains biological activity as a DRG growth cone collapsing agent and saturably binds to DRG neurons with low nanomolar affinity. Specific CAP-4 binding sites are present on DRG neurons, sy mpathetic neurons, and motoneurons, but not on retinal, cortical, or b rainstem neurons. Outside the nervous system, high levels of CAP-4 bin ding sites are present in the mesenchyme surrounding major blood vesse ls and developing bone and in lung. These sites provide a substrate fo r the collapsin-1-dependent patterning of non-neuronal tissues perturb ed in sema III (-/-) mice. The staining patterns for mouse semaphorin D/III and chick collapsin-1 fusion proteins are indistinguishable from one another but quite separate from that for semaphorin B and M-semap horin F fusion proteins. These data imply that a family of high-affini ty semaphorin binding sites similar in complexity to the semaphorin li gand family exists.