New insights into PKC family affairs: three novel phosphorylation sites in PKCepsilon and at least one is regulated by PKCalpha.

PKCepsilon (protein kinase Cepsilon) is a serine/threonine kinase, and a member of the PKC family of isoforms. The different PKC isoforms regulate many cellular processes of importance for disease. It is therefore desirable to obtain tools to specifically modulate the activity of the individual isoforms and to develop markers of PKC activity. The paper by Durgan et al. in this issue of the Biochemical Journal has taken us some steps further towards these goals. In the paper they identify three previously unknown phosphorylation sites in PKCepsilon. All of them are specific for the epsilon isoform, evolutionarily conserved and tightly regulated. The phosphorylation of one site is critical for the binding of PKCepsilon to 14-3-3beta, suggesting it is of functional importance. The results provide important novel findings that uncover new aspects of PKCepsilon regulation and reveal new possibilities for detecting PKCepsilon activity in situ.

The identification and characterization of novel PKCepsilon phosphorylation sites provide evidence for functional cross-talk within the PKC superfamily.

PKCepsilon (protein kinase Cepsilon) is a phospholipid-dependent serine/threonine kinase that has been implicated in a broad array of cellular processes, including proliferation, survival, migration, invasion and transformation. Here we demonstrate that, in vitro, PKCepsilon undergoes autophosphorylation at three novel sites, Ser(234), Ser(316) and Ser(368), each of which is unique to this PKC isoform and is evolutionarily conserved. We show that these sites are phosphorylated over a range of mammalian cell lines in response to a number of different stimuli. Unexpectedly, we find that, in a cellular context, these phosphorylation events can be mediated in-trans by cPKC (classical PKC) isoforms. The functional significance of this cross-talk is illustrated through the observation that the cPKC-mediated phosphorylation of PKCepsilon at residue Ser(368) controls an established PKCepsilon scaffold interaction. Thus our current findings identify three new phosphorylation sites that contribute to the isoform-specific function of PKCepsilon and highlight a novel and direct means of cross-talk between different members of the PKC superfamily.