Regulating the regulator: Insights into the cardiac protein phosphatase 1 interactome

Abstract

Reversible phosphorylation of proteins is a delicate yet dynamic balancing act between kinases and phosphatases, the disturbance of which underlies numerous disease processes. While our understanding of protein kinases has grown tremendously over the past decades, relatively little is known regarding protein phosphatases. This may be because protein kinases are great in number and relatively specific in function, and thereby amenable to be studied in isolation, whereas protein phosphatases are much less abundant and more nonspecific in their function. To achieve subcellular localization and substrate specificity, phosphatases depend on partnering with a large number of regulatory subunits, protein scaffolds and/or other interactors. This added layer of complexity presents a significant barrier to their study, but holds the key to unexplored opportunities for novel pharmacologic intervention. In this review we focus on serine/threonine protein phosphatase type-1 (PP1), which plays an important role in cardiac physiology and pathophysiology. Although much work has been done to investigate the role of PP1 in cardiac diseases including atrial fibrillation and heart failure, most of these studies were limited to examining and manipulating the catalytic subunit(s) of PP1 without adequately considering the PP1 interactors, which give specificity to PP1's functions. To complement these studies, three unbiased methods have been developed and applied to the mapping of the PP1 interactome: bioinformatics approaches, yeast two-hybrid screens, and affinity-purification mass spectrometry. The application of these complementary methods has the potential to generate a detailed cardiac PP1 interactome, which is an important step in identifying novel and targeted pharmacological interventions.

Keywords: Atrial fibrillation; Heart failure; Protein phosphatase; Proteomics; Regulatory subunit.

Figure 1

Sequence alignment of human and mouse PP1 isoforms using Clustal Omega (http://www.ebi.ac.uk/Tools/msa/clustalo/).

Figure 2

The cardiac PP1 interactome. Modified from Chiang et al.[9] and incorporated data from various studies as well as from the Human Protein Atlas (http://www.proteinatlas.org/). All known PP1 regulatory (R)-subunits or interactors (yellow) in the heart are listed in Table 1.