Cyclic AMP‐Dependent Protein Phosphorylation in Isolated Neuronal Growth Cones from Developing Rat Forebrain

We have shown recently that neuronal growth cones isolated from developing rat forebrain possess an appreciable activity of adenylate cyclase, which produces cyclic AMP and can be stimulated by various neurotransmitter receptor agonists and by forskolin. To investigate cyclic AMP‐mediated biochemical mechanisms in isolated growth cones, we have centered the present study on cyclic AMP‐dependent protein phosphorylation. One‐dimensional gel electrophoretic analysis showed that cyclic AMP analogs increased incorporation of 32 P into several phosphoproteins in molecular mass ranges of 50–58 and 76–82 kilodaltons, including those of 82, 76, and 51 kilodaltons. Two‐dimensional electrophoresis, using isoelectric focusing in the first dimension, resolved phosphorylated α ‐ and β ‐tubulin species, actin, a very acidic protein (isoelectric point 4.0) with a molecular mass of 93 kilodaltons, and two proteins (x and x′) closely neighboring β ‐tubulin. Two other phosphoproteins seen in the gels had molecular masses of 56 and 51 kilodaltons (respective isoelectric points, 4.5 and 4.4) and, along with the 93‐kilodalton phosphoprotein, were highly enriched in the isolated growth cones. Only the tubulin and actin species were major proteins in the isolated growth cones. Cyclic AMP analogs enhanced incorporation of 32 P into phosphoproteins x and x′, and, as assessed by immunoprecipitation, into β ‐tubulin. Peptide digest experiments suggested that phosphoproteins x and x′ are unrelated to β ‐tubulin. Nonequilibrium two‐dimensional electrophoresis resolved many phosphoproteins, of which a 79‐ and 75‐kilodalton doublet, a 74‐kilodalton species, and a 58‐kilodalton doublet showed enhanced incorporation of 32 P in the presence of cyclic AMP.