Abstract
Cyclic adenosine 3′:5′-monophosphate (cAMP)-binding proteins and cAMP- dependent protein phosphorylation were examined in growth cone particles (GCPs) prepared from fetal rat brain. Several major proteins which specifically bind a photoactivatable analogue of cAMP are observed in GCPs and correspond to isoelectric variants of the regulatory subunits of the cAMP-dependent protein kinase described in adult brain. We found no evidence for differential compartmentalization of specific cAMP-binding proteins in subcellular fractions of fetal brain or within GCPs. cAMP-stimulated phosphoproteins of GCPs are similar to cAMP-dependent protein kinase substrates characterized in nerve terminals (synaptosomes) of adult brain and include the nerve terminal-specific protein, synapsin I. However, as shown in the companion paper (Katz, F., L. Ellis, and K. H. Pfenninger (1985) J. Neurosci. 5: 1402–1411), this synaptic phosphoprotein is not the major kinase substrate in the GCP fraction. The finding of synapsin I in a subcellular fraction prepared from fetal brain suggests that components of the mature nerve terminal are already present in fetal brain during neuronal sprouting and prior to synaptogenesis.