bFGF stimulates GAP-43 phosphorylation at ser41 and modifies its intracellular localization in cultured hippocampal neurons.

Cultured hippocampal neurons have been used to study GAP-43 phosphorylation and subcellular distribution. By immunofluorescence, GAP-43 was found associated with adherent membrane patches that remained attached to the substratum after in situ permeabilization with Nonidet-NP40. This association increases during neuronal development and is stabilized by the actin cytoskeleton. Basic fibroblast growth factor (bFGF) promotes GAP-43 translocation from the cytosol to adherent membrane patches and, at the same time, stimulates GAP-43 phosphorylation, mainly at the protein kinase C (PKC) site (Ser41). Inhibition of PKC prevented bFGF-stimulated GAP-43 phosphorylation and translocation, while activation by phorbol esters mimicked bFGF effects, suggesting that phosphorylation at Ser41 regulates GAP-43 subcellular localization. Using biochemical fractionation and phosphorylation analysis, it was found that Ser41 phosphorylation was highest in cytoskeleton-associated GAP-43 and lowest in membrane-associated GAP-43. It is proposed that GAP-43 is continuously cycling between intracellular compartments depending on its phosphorylation state and could be taking part in initial adhesive complexes assembled during growth cone advance.