Nerve growth cones isolated from fetal rat brain. III. Calcium-dependent protein phosphorylation.

Calcium-dependent protein kinase activities have been studied in nerve growth cone particles (GCPs) and compared with those of synaptosomes. GCPs contain a set of phosphoproteins qualitatively similar to that of synaptic nerve terminals. However, major quantitative differences appear to exist: whereas synapsin I phosphorylation is relatively weak, the major kinase substrates of GCPs are a 46,000-dalton membrane protein (calcium/calmodulin dependent) and two acidic proteins of 80,000 and 40,000 daltons, phosphorylated by a calcium/phospholipid-dependent protein kinase. The presence of synaptic kinase activities in GCPs is consistent with their neuronal origin. The role of these kinases in GCPs is not understood at present. They may be involved in growth-related functions and/or may prepare the sprouting neuron for synaptic function.