Cardiac myofilament regulation by protein phosphatase type 1α and CapZ

Myofilament regulation by protein kinases is well characterized, but relatively little is known about protein phosphatase control of myofilaments. Increased protein phosphatase type 1 (PP1) activity observed in failing hearts underscores the need for investigation of this intracellular signal, including the elements that regulate its activity. The Z-disc protein CapZ controls protein kinase C (PKC) regulation of cardiac myofilaments, but whether this effect is specific to PKC, or CapZ plays a general role in intracellular signalling, is not known. We sought to determine how the alpha isoform of PP1 (PP1 alpha) regulates murine cardiac myofilaments and whether CapZ influences PP1 alpha-dependent regulation of cardiac myofilaments. Immunoblot analysis showed PP1 alpha binding to cardiac myofilaments. Exogenous PP1 alpha increased myofilament Ca2+ sensitivity and maximal actomyosin MgATPase activity while dephosphorylating myosin binding protein C, troponin T, troponin I, and myosin light chain 2. Extraction of CapZ decreased myofilament-associated PP1 alpha and attenuated the effects of PP1 alpha on myofilament activation. PP1 alpha-dependent dephosphorylation of myofilament proteins was reduced with CapZ extraction, except for troponin I. Extracting CapZ after PP1 alpha treatment allowed most of the PP1 alpha-dependent effects on myofilament activation to remain, indicating that CapZ removal modestly desensitizes cardiac myofilaments to dephosphorylation. Our results demonstrate myofilament regulation by PP1 alpha and support the concept that cardiac Z-discs are vital components in intracellular signalling.