B-50 (GAP-43): biochemistry and functional neurochemistry of a neuron-specific phosphoprotein.

The biochemistry and functional neurochemistry of the synaptosomal plasma membrane phosphoprotein B-50 (GAP-43) are reviewed. The protein is putatively involved in seemingly diverse functions within the nervous system, including neuronal development and regeneration, synaptic plasticity, and formation of memory and other higher cognitive behaviors. There is a considerable amount of information concerning the spatial and temporal localization of B-50 (GAP-43) in adult, fetal, and regenerating nervous tissue but far less is known about the physical chemistry and biochemistry of the protein. Still less information is available about posttranslational modifications of B-50 (GAP-43) that may be the basis of neurochemical mechanisms that could subsequently permit a variety of physiological functions. Hence, consideration is given to several plausible roles for B-50 (GAP-43) in vivo, which are discussed in the context of the cellular localization of the protein, significant posttranslational enzymes, and regulatory proteins, including protein kinases, phosphoinositides, calmodulin, and proteases.