Editors' ChoiceProtein Interactions

Direct and Indirect Inhibition of PKC

STKE  25 Mar 2003:
Vol. 2003, Issue 175, pp. tw120-TW120
DOI: 10.1126/stke.2003.175.tw120

Protein kinase K (PKC) is activated by the lipid diacylglycerol (DAG). DAG is metabolized by DAG kinases (DGKs), which convert DAG to phophatidic acid. Thus, DGKs control PKC activity indirectly by eliminating an allosteric activator, DAG. Both DGKs and PKCs exist as large families with variable domains involved in localization and regulation of these enzymes. Luo et al. report a novel direct mechanism for inhibition of PKCα by DGKζ, which has a region homologous to the phosphorylation site domain of the myristoylated alanine-rich C kinase substrate (MARCKS) protein. MARCKS is a substrate for PKC, and phosphorylation of the MARCKS domain of DGKζ by PKC reduces nuclear localization of DGKζ. DGKζ and PKCα coprecipitated from transfected cells, as did endogenous proteins expressed in rat brain extracts and nontransfected cells. The interaction was mediated by a portion of the catalytic domain of DGKζ. Activation of PKC by treatment of cells or recombinant PKC with phorbol esters inhibited the interaction of PKC and DGKζ. Activation of PKC by growth factor stimulation of cells also inhibited the coprecipitation of DGK activity. Mutations that mimicked phosphorylation of the MARCKS domain of DGKζ prevented the interaction with PKC, and stimulation of PKC with phorbol ester did not block the interaction between PKC and a mutant of DGKζ with a nonphosphorylatable MARCKS domain. Thus, phosphorylation of the MARCKS domain by PKC appears to represent one mechanism controlling the interaction between these two proteins. The importance of phosphorylation of the MARCKS domain in controlling not just protein interaction, but also DGKζ inhibition of PKCα activity was confirmed by in vitro kinase assays of PKC immunoprecipitated from cells expressing wild-type, phosphorylation-mimic versions of DGKζ, or nonphosphorylatable versions of DGKζ. The authors propose a model whereby in the basal state, the DGKζ-PKCα complex holds PKC in an inactive state and DGKζ phosphorylates DAG, keeping the system off. Upon stimulation, DAG levels rise above that which DGKζ can metabolize, stimulating PKCα activity. This results in the phosphorylation and release of DGKζ, further promoting PKCα activity.

B. Luo, S. M. Prescott, M. K. Topham, Association of diacylglycerol kinase ζ with protein kinase C α: Spatial regulation of diacylglycerol signaling. J. Cell Biol. 160, 929-937 (2003). [Abstract] [Full Text]