Control of cardiac myofilament activation and PKC-βII signaling through the actin capping protein, CapZ

Actin capping protein (CapZ) anchors the barbed ends of sarcomeric actin to the Z-disc. Myofilaments from transgenic mice (TG-CapZ) expressing a reduced amount of CapZ demonstrate altered function and protein kinase C (PKC) signaling [Pyle WG, Hart MC, Cooper JA, Sumandea MP, de Tombe PP, and Solaro RJ., Circ. Res. 90 (2002) 1299-306]. The aims of the current study were to determine the direct effects of CapZ on myofilament function and on PKC signaling to the myofilaments. Our studies compared mechanical properties of single myocytes from TG-CapZ mouse hearts to wild-type myocytes from which CapZ was extracted using PIP2. We found that myofilaments from CapZ-deficient transgenic myocardium exhibited increased Ca2+ sensitivity and maximum isometric tension. The extraction of CapZ from wild-type myofilaments replicated the increase in maximum isometric tension, but had no effect on myofilament Ca2+ sensitivity. Immunoblot analysis revealed that the extraction of CapZ was associated with a reduction in myofilament-associated PKC-βII and that CapZ-deficient transgenic myofilaments also lacked PKC-βII. Treatment of wild-type myofilaments with recombinant PKC-βII reduced myofilament Ca2+ sensitivity, whereas this effect was attenuated in myofilaments from TG-CapZ mice. Our results indicate that cardiac CapZ directly controls maximum isometric tension generation, and establish CapZ as an important component in anchoring PKC-βII at the myofilaments, and for mediating the effects of PKC-βII on myofilament function.