Protein kinase Mζ

Protein kinase Mζ (also called PKMζ or PKMzeta) is the independent catalytic domain of protein kinase Cζ and, lacking an autoinhibitory regulatory domain of the full-length PKCζ, is constitutively active. This constitutive or autonomous activity allows the kinase to be independent of second messengers and thus persistently active. It was originally thought of as being a cleavage product of full-length PKCζ, an atypical isoform of protein kinase C (PKC). Like other PKC isoforms, PKCζ is a serine/threonine kinase that adds phosphate groups to target proteins. It is atypical in that unlike other PKC isoforms, PKCζ does not require calcium or diacylglycerol (DAG) to become active, but rather relies on a second messenger other than DAG, presumably generated through a phosphoinositide 3-kinase (PI3-kinase) pathway. It is now known that PKMζ is not the result of cleavage of full-length PKCζ, but rather, in mammalian brain, is translated from its own brain-specific mRNA, that is transcribed from the full-length PKCζ gene. The promotor for full-length PKCζ is largely inactive in the forebrain and so PKMζ is the dominant form of ζ in the forebrain and the only PKM that is translated from its own mRNA.

PKMζ is thought to be responsible for maintaining the late phase of long-term potentiation. This theory arose from the observation that PKMζ perfused postsynaptically into neurons causes synaptic potentiation and selective inhibitors of PKMζ, when bath applied 1 hr after tetanization, inhibit the late phase or maintenance of LTP. Thus PKMζ is both necessary and sufficient for maintaining LTP. Subsequent work showed that inhibiting the kinase reversed LTP maintenance when applied up to 5 hours after LTP was induced in hippocampal slices, and after 22 hr in vivo. Inhibiting PKMζ in behaving animals erased spatial long-term memories in the hippocampus that were up to 1 month-old, without affecting spatial short-term memories. In the neocortex, thought to be the site of storage for most long-term memories, PKMζ inhibition erased memories for conditioned taste aversion in the insular cortex. PKMζ is thus the first molecule shown to be a component of the storage mechanism of long-term memory.