Serotonin-Induced Cleavage of the Atypical Protein Kinase C Apl III in Aplysia

A constitutively active kinase, known as protein kinase M zeta (PKM zeta), is proposed to act as a long-lasting molecular memory trace. While PKM zeta is formed in rodents through translation of a transcript initiating in an intron of the protein kinase C zeta (PKC zeta) gene, this transcript does not exist in Aplysia californica despite the fact that inhibitors of PKM zeta erase memory in Aplysia in a fashion similar to rodents. We have previously shown that, in Aplysia, the ortholog of PKC zeta, PKCApl III, is cleaved by calpain to form a PKM after overexpression of PKC Apl III. We now show that kinase activity is required for this cleavage. We further use a FRET reporter to measure cleavage of PKC Apl III into PKM Apl III in live neurons using a stimulus that induces plasticity. Our results show that a 10 min application of serotonin induces cleavage of PKC Apl III in motor neuron processes in a calpain-and protein synthesis-dependent manner, but does not induce cleavage of PKC Apl III in sensory neuron processes. Furthermore, a dominant-negative PKM Apl III expressed in the motor neuron blocked the late phase of intermediate-term facilitation in sensory-motor neuron cocultures induced by 10 min of serotonin. In summary, we provide evidence that PKC Apl III is cleaved into PKM Apl III during memory formation, that the requirements for cleavage are the same as the requirements for the plasticity, and that PKM in the motor neuron is required for intermediate-term facilitation.