Protein kinase C phosphorylated at a conserved threonine is retained in the cytoplasm

Phosphorylation of calcium-activated protein kinase Cs (PKCs) at threonine 634 and/or threonine 641 increases during long term potentiation or associa tive learning in rodents. In the marine mollusk Aplysia, persistent activat ion of the calcium-activated PKC Apl I occurs during long term facilitation . We have raised an antibody to a peptide from PKC Apl I phosphorylated at threonines 613 and 620 (sites homologous to threonines 634 and 641). This a ntibody recognizes PKC Apl I only when it is phosphorylated at threonine 61 3. Both phorbol esters and serotonin increase the percentage of kinase phos phorylated at threonine 613 in Aplysia neurons. Furthermore, the pool of PK C that is phosphorylated at threonine 613 in neurons is resistant to both m embrane translocation and down-regulation. Replacement of threonine 613 wit h alanine increased the affinity of PKC Apl I for calcium, suggesting that phosphorylation of this site may reduce the ability of PKC Apl I to translo cate to membranes in the presence of calcium. We propose that phosphorylati on of this site is important for removal of PKC from the membrane and may b e a mechanism for negative feedback of PKC activation.