The accurate extension of neuronal processes during development is essential to the proper functioning of the mature nervous system. The long term goal of this research is to understand how molecules on axons promote, and direct, the extension of other axons. The specific objective of this proposal is to study the molecular and functional properties of an axonal surface molecule that may function in promoting neurite extension. A novel cell surface molecule has been identified and named DM-RASP. I have immunopurified DM and it is a single protein species of 95 kD. Its amino terminal protein sequence has been determined, and by comparison to the Genpept database it is unique. Oligonucleotide probes designed from the amino acid analysis have identified five clones from an E19 chick brain CDNA library. One of these clones has been partially sequenced, and the deduced amino acids from the partial sequence show that DM-RASP is a member of the immunoglobulin superfamily of neural cell adhesion molecules. Northern analysis of chick brain RNA using as a probe the CDNA insert from one of the clones reveals two DM-RASP messages. Antibodies against DM-RASP reduce neurite extension on axons, suggesting that its function is to support axon extension. DM-RASP is expressed on the surface of some but not all early axons in the developing chick embryo. Thus it is possible that DM-RASP may be involved in helping axons make pathway specific choices. The first objective is to completely sequence the message for DM-RASP. Southern analysis will show whether the DM-RASP MRNAS are the result of two different genes or the result of alternate processing of one gene. A developmental profile of DM-RASP gene expression will be obtained by Northern analysis, and this will also show whether the multiple messages are differentially regulated. The second objective is to study the functional properties of DM-RASP. The amount of DM-RASP required to support neurite extension will be determined. The neuronal cell types which extend upon DM-RASP will be identified. Neuronal growth cones will be cultured on alternate stripes of DM-RASP and other proteins, to test if growth cones show a preference for one substrate molecule versus another. If a cell specific choice to extend upon DM-RASP is observed, then it could be that DM-RASP acts to specifically direct neurite extension. The third objective is to identify the mouse homologue of DM-RASP and other related proteins. Since we have identified one vertebrate cell surface molecule with a restricted distribution in the nervous system, we may now be able to identify others.