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Growth cones are specialized structures that form the distal tips of growing axons. During both normal development of the nervous system and regeneration of injured nerves, growth cones are essential for elongation and guidance of growing axons. Developmental and regenerative axon growth is frequently accompanied by elevated synthesis of a protein designated GAP-43. GAP-43 has now been found to be a major component of growth-cone membranes in developing rat brains. Relative to total protein, GAP-43 is approximately 12 times as abundant in growth-cone membranes as in synaptic membranes from adult brains. Immunohistochemical localization of GAP-43 in frozen sections of developing brain indicates that the protein is specifically associated with neuropil areas containing growth cones and immature synaptic terminals. The results support the proposal that GAP-43 plays a role in axon growth.

Nerve regeneration and developmental outgrowth of axons are both correlated with increased synthesis of an axonal membrane protein designated GAP-43. Phosphorylation of an apparently identical protein, present at lower abundance in adult brains, has been correlated with long-term potentiation, a form of synaptic plasticity. We have now isolated a cDNA clone encoding GAP-43 from neonatal rat brain. The amino acid sequence is extremely hydrophilic, with no potential membrane-spanning domains and no sites for N-linked glycosylation, but with a short hydrophobic segment at the protein's amino terminus, consistent with a model in which GAP-43 extends from the cytoplasmic surface of growth cone and synaptic plasma membranes. Among several tissues and cells examined, GAP-43 mRNA is expressed only in neurons. Developmental and regeneration-associated changes in GAP-43 synthesis appear to be mediated largely at the level of transcription of a single gene.