Differential phosphorylation of neuronal substrates by catalytic subunits of Aplysia cAMP-dependent protein kinase with alternative N termini.

cAMP-dependent protein kinase (PKA) is an important participant in neuronal modulation: the ability of neurons to change their properties in response to external stimuli. In Aplysia mechanosensory neurons, PKA plays roles in both short and long term presynaptic facilitation, which is a simple model for learning and memory. PKA in Aplysia is a collection of structurally and functionally diverse regulatory and catalytic (C) subunits. We have argued that this diversity may in part account for the ability of the enzyme to take part in neuronal events that are spatially and temporally separated. Here, we add credence to this hypothesis by showing that C subunits of Aplysia PKA with alternative N termini target different substrates in subcellular fractions from Aplysia neurons, despite their similar actions on synthetic peptide substrates. Purified recombinant CAPL-AN1A1, which has an N terminus that is homologous to the myristylated sequence described in mammals, catalyzes the formation of two phosphoproteins of 24 and 8 kDa more rapidly than CAPL-AN2A1, which has a distinct N terminus weakly related to that of the yeast TPK1 gene product. The 24-kDa phospoprotein, but not the 8-kDa species, is detected in taxol-stabilized microtubules, suggesting that it is associated with the cytoskeleton. CAPL-AN2A1, in contrast, generates a 55-kDa phosphoprotein that is not observed with CAPL-AN1A1. The 55-kDa species is found in the detergent supernatant of the cytoskeleton fraction. Differential targeting of substrates by C subunits of PKA may therefore contribute to the ability of this kinase to play multiple roles in neuronal modulation.