Phosphoproteins associated with the regulation of a specific potassium channel in the identified Aplysia neuron R15.

The neurotransmitter serotonin (5HT) activates a specific K+ conductance in the identified Aplysia neuron R15. This response to 5HT has been shown previously to be mediated by cAMP and cAMP-dependent protein phosphorylation. We have measured protein phosphorylation within neuron R15 in vivo, following the intracellular injection of [gamma-32P]ATP, and have demonstrated that 5HT modulates the phosphorylation of a number of proteins in R15. The present study was undertaken to determine which of these phosphoproteins are closely associated with, and may be responsible for, the K+ conductance increase. Treatment of neuron R15 with a cAMP analog produces some but not all of the 5HT-induced phosphoprotein changes, indicating that some are not cAMP-dependent and thus can be dissociated from the cAMP-dependent K+ conductance increase. Similar results are obtained by intracellular injection of the adenylate cyclase inhibitor guanosine 5'-O-(2-thiodiphosphate), which completely blocks the 5HT-evoked K+ conductance increase but fails to block some of the 5HT-induced phosphorylation changes. Examination of the phosphoprotein pattern at short times after 5HT application has demonstrated that some of the phosphoprotein changes, but not others, are closely associated in time with the appearance of the physiological response. These and other pharmacological and kinetic experiments have allowed the identification of two phosphoproteins, of Mr = 29,000 and 70,000, which cannot be dissociated from the 5HT-induced K+ conductance increase whatever the experimental manipulation. Thus, one or both of these phosphoproteins may be involved in the regulation of the 5HT-sensitive K+ channel in neuron R15.