title: GAP-43 as a 'calmodulin sponge' and some implications for calcium signalling in axon terminals.
abstract: In most neurons, the maturation of axonal growth cones to become stable synaptic terminals is accompanied by a dramatic decline in the abundance of a major growth cone component, GAP-43. Accumulation of GAP-43 persists, however, in a minority of mature synaptic terminals. What properties of axons and their terminals are affected by these changes in GAP-43 expression? Storm and colleagues first noted that the membrane- and calmodulin-binding properties of GAP-43 (a.k.a. P-57 or neuromodulin) could allow it to sequester a large fraction of calmodulin to the submembranous regions, and to release free calmodulin in response to protein kinase C activation. Analysis of evolutionarily conserved sequences in GAP-43 indicates that these properties are central to the biological effects of the protein. If GAP-43 is presumed to inactivate bound calmodulin, the network of GAP-43 in an axon terminal could be considered a regulatable calmodulin buffer, or 'calmodulin sponge', absorbing free calmodulin and releasing it in response to activation of protein kinase C. Such a calmodulin sponge has properties that could be useful in modulating the responses of membrane and cytoskeletal assembly events to calcium signals in growth cones, and in mediating long-term potentiation of neurotransmitter release from some pre-synaptic terminals.
authors.affiliation: Department of Neurobiology, Stanford University, California.
authors.forename: J H
authors.initials: JH
authors.lastname: Skene
mesh_terms: D000818:Animals; D001369:Axons / Q000502:physiology*; D002118:Calcium / Q000502:physiology*; D002147:Calmodulin / Q000378:metabolism*; D019922:GAP-43 Protein; D006133:Growth Substances / Q000502:physiology; D008562:Membrane Glycoproteins / Q000502:physiology*; D008959:Models, Neurological; D009411:Nerve Endings / Q000502:physiology; D009419:Nerve Tissue Proteins / Q000502:physiology*; D018377:Neurotransmitter Agents / Q000378:metabolism; D015398:Signal Transduction; D013569:Synapses / Q000502:physiology; D009435:Synaptic Transmission / Q000502:physiology
chemical_list: D002147:Calmodulin; D019922:GAP-43 Protein; D006133:Growth Substances; D008562:Membrane Glycoproteins; D009419:Nerve Tissue Proteins; D018377:Neurotransmitter Agents; D002118:Calcium