Axonin-I is an axon-associated cell adhesion molecule (AxCAM) of the chicken, which promotes neurite outgrowth by interaction with the AxCAM Ll(G4) of the neuritic membrane. Here we report the cloning and sequence determination of a cDNA encoding axonin-I, Peptides generated by enzymatic cleavage showed similarity to the AxCAM F11. Degenerated polymerase chain reaction (PCR) primers were designed and an axonin-I fragment was amplified from mRNA of embryonic retina. Screening of a cDNA library from embryonic brain resulted in the isolation of a 4.0-kb cDNA insert with an open reading frame of 3108 nucleotides. The deduced polypeptide of 1036 amino acids includes a putative hydrophobic N-terminal signal sequence of 23 or 25 amino acids and a C-terminal hydrophobic sequence of 29 amino acids which is suggestive of sequences serving as signal for the attachment of a glycosyl-phosphatidylinositol (glycosyl-PtdIns) anchor. The putative mature form of axonin-1 comprises six immunoglobulin-like repeats, followed by four fibronectin-type I11 repeats.Axonin-1 exhibits 75% amino acid identity with the AxCAM TAG-1 of the rat, suggesting that it is the chicken homologue of TAG-1. Like TAG-1, axonin-I is glycosyl-PtdIns-anchored to the neuronal membrane; in contrast to TAG-1, it does not exhibit an Arg-Gly-Asp sequence.The interconnection of neurons by processes forming synaptic contacts is one of the crucial developmental stages of neurogenesis. In order to span long distances, bundles of axons are formed in a process which is thought to be driven by the tendency of growing axons to adhere specifically to and elongate along preexisting axons. The consistency with which

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