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A B56 regulatory subunit of protein phosphatase 2A localizes to nuclear speckles in cardiomyocytes

¹Department of Biochemistry and Molecular Biology, University of Maryland School of Medicine, Baltimore, Maryland; and ²Department of Pathology, Medical School, and ³Department for Molecular Genetics, Research Institute for Microbial Diseases, Osaka University, Suita, Osaka, Japan

Submitted 22 December 2004 ; accepted in final form 7 March 2005

Protein phosphatase 2A (PP2A) is widely distributed in heart tissues, yet its precise cellular functions are poorly understood. This study is based on the notion that PP2A action is governed by interactions of the core enzyme with B targeting/regulatory subunits. The subcellular localizations of two B subunits, B56 and B561, were assessed using adenovirus-driven expression of epitope-tagged (hemagglutinin, HA) in cultured neonatal and adult rat ventricular myocytes. Confocal imaging revealed that HA-B56 was excluded from the nucleus and decorated striated structures, whereas HA-B561 was principally found in the nucleus. Precise immunolabeling studies showed that B561 was concentrated in intranuclear structures known as nuclear speckles, macromolecular structures that accumulate transcription and splicing factors. Western blot analyses revealed that overexpression of either B subunit had no effect on the levels of other PP2A subunits in cultured neonatal cardiac cells. However, overexpression of only B561 increased whole cell PP2A activity by 40% when measured in cell extracts. Finally, B561 did not alter global gene expression or expression of hypertrophic gene markers such as -skeletal actin. However, morphometric analyses of confocal images revealed that B561 alters the dynamic assembly/disassembly process of nuclear speckles in heart cells. These studies provide new insight into mechanisms of PP2A targeting in the subnuclear architecture in cardiomyocytes and into the role of this phosphatase in nuclear signaling.

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Corrigendum

AJP - Heart 2005 289: H2753. [Full Text]  

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