Localization and kinetics of protein kinase C-epsilon anchoring in cardiac myocytes

Protein kinase C-epsilon (PKC-epsilon) plays a central role in cardiac cell signaling, but mechanisms of translocation and anchoring upon activation are poorly understood. Conventional PKC isoforms rely on a rapid Ca2+-mediated recruitment to cell membranes, but this mechanism cannot be employed by PKC-epsilon or other PKC isoforms lacking a Ca2+-binding domain. In this study, we used recombinant green fluorescent protein (GFP) fusion constructs and confocal microscopy to examine the localization, kinetics, and reversibility of PKC-epsilon anchoring in permeabilized rat cardiac myocytes. PKC-epsilon -GFP bound with a striated pattern that co-localized with alpha -actinin, a marker of the Z-line of the sarcomere. Binding required activation of PKC and occurred slowly but reversibly with apparent rate constants of k(on) = 4.6 +/- 1.2 x 10(3) M-1 s(-1) and k(off) = 1.4 +/- 0.5 x 10(-3) s(-1) (t(1/2) = 8 min) as determined by fluorescence recovery after photobleaching and by perfusion experiments. A truncated construct composed of the N-terminal 144-amino-acid variable region of PKC-epsilon (epsilonV(1)-GFP), but not an analogous N-terminal domain of PKC-delta, mimicked the Z-line decoration and slow binding rate of the full-length enzyme. These findings suggest that the epsilonV(1) domain is important in determining PKC-epsilon localization and translocation kinetics in cardiac muscle. Moreover, PKC-epsilon translocation is not a diffusion-controlled binding process but instead may be limited by intramolecular conformational changes within the V-1 domain. The k(off) for epsilonV(1)-GFP was two- to threefold faster than for full-length enzyme, indicating that other domains in PKC-epsilon contribute to anchoring by prolonging the bound state.