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 neu ronal 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, whic h is a simple model for learning and memory. PKA in Aplysia is a colle ction of structurally and functionally diverse regulatory and catalyti c (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 ar e spatially and temporally separated. Here, we add credence to this hy pothesis by showing that C subunits of Aplysia PRA with alternative N termini target different substrates in subcellular fractions from Aply sia neurons, despite their similar actions on synthetic peptide substr ates. Purified recombinant C-APL-A N1A1, which has an N terminus that is homologous to the myristylated sequence described in mammals, catal yzes the formation of two phosphoproteins of 24 and 8 kDa more rapidly than C-APL-A N2A1, which has a distinct N terminus weakly related to that of the yeast TPK1 gene product. The 24-kDa phosphoprotein, but no t the 8-kDa species, is detected in taxol-stabilized microtubules, sug gesting that it is associated with the cytoskeleton. C-APL-A N2A1, in contrast, generates a 55 kDa phosphoprotein that is not observed with C-APL-A N1A1. 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 kin ase to play multiple roles in neuronal modulation.