Cp20 is a neuronal protein involved in the molecular mechanisms of synaptic plasticity. Cp20 was first identified in the CNS of the marine snail Hermissenda, which can be trained to learn an association between visual and vestibular stimuli in a Pavlovian conditioning procedure. Upon learning, cp20 is phosphorylated in a few defined neurons of Hermissenda, and inhibits the Ca2+-dependent K+ channels. This results in an increase in membrane resistence and therefore a hyperexcitability of the neuronal pathway. Similar biochemical mechanisms have been proposed to underlie memory in higher species, such as rabbit and rat. Cp20 was isolated from squid optic lobes by a 5-step purification. The protein was cloned, fused with an oligohistidine tail, and expressed in Esterichia coli. The activation of PKC, the enzyme responsible for cp20 phosphorylation, was also studied by an in vivo imaging system.