Protein phosphatases 1 and 2A and their naturally occurring inhibitors: current topics in smooth muscle physiology and chemical biology

Abstract

Protein phosphatases 1 and 2A (PP1 and PP2A) are the most ubiquitous and abundant serine/threonine phosphatases in eukaryotic cells. They play fundamental roles in the regulation of various cellular functions. This review focuses on recent advances in the functional studies of these enzymes in the field of smooth muscle physiology. Many naturally occurring protein phosphatase inhibitors with different relative PP1/PP2A affinities have been discovered and are widely used as powerful research tools. Current topics in the chemical biology of PP1/PP2A inhibitors are introduced and discussed, highlighting the identification of the gene cluster responsible for the biosynthesis of calyculin A in a symbiont microorganism of a marine sponge.

Keywords: Phosphatase inhibitors; Protein phosphatase 1; Protein phosphatase 2A; Protein phosphorylation; Signal transduction; Smooth muscle.

Fig. 1

Current scheme for the regulation of smooth muscle contraction. GPCR G-protein coupled receptor; PLCβ phospholipase Cβ; PIP ₂ phosphatidylinositol 4,5-bisphosphate; IP ₃ inositol 1,4,5-tripsphosphate; DAG diacylglycerol; PKC protein phosphatase C; SR sarcoplasmic reticulum; CaM calmodulin; MLCK myosin light chain kinase; MLCP myosin light chain phosphatase; GEF guanine nucleotide exchange factor; ROCK RhoA/rho-associated coiled-coil-containing protein kinase; CPI-17 PKC-potentiated inhibitory protein of 17 kDa. See the text for details

Fig. 2

The structure of the myosin light chain phosphatase (MLCP) and that of the endogenous regulatory protein CPI-17. PP1 a type-1 phosphatase catalytic subunit (PP1; β/δ isoform), MYPT1 myosin phosphatase target subunit 1, 8× ANK 8-repeat ankyrin motif, CI central insert domain, LZ Leu-zipper domain, CC coiled-coil domain, M20 the 20-MDa accessory subunit, LMM light meromyosin. See also the legend to Fig. 1

Fig. 3

Schematic presentation of the mechanism of the vascular intrinsic clock that generates the circadian rhythm of the activity of myosin light chain phosphatase (MLCP). The circadian oscillation of the expression of the clock gene RORα is translated to the rhythmic changes in the expression and activity of ROCK2, which in turn causes the oscillatory changes in the phosphorylation of MYPT1, a regulatory subunit of MLCP, at Thr853 (pT853) and the phosphorylation of MCL (MLC-P) induced by receptor agonist, thereby generating diurnal changes in the vascular contractility. The processes, which exhibited a circadian rhythm in [20], are indicated by clock images. MLCP is composed of three subunits: a catalytic subunit PP1, MYPT1 and another regulatory subunit M20. The activity of MLCP is negatively regulated by the phosphorylation of MYPT1 at either Thr696 (pT696) or T853 (pT853). Therefore, when the expression of ROCK2 and the phosphorylation of MYPT1 are high (solid line curve), the activity of MLCP is considered to be reciprocally low (dashed line curve). MLC 20-kDa myosin light chain, RORα retinoic acid receptor-related orphan receptor α (see also Figs. 1, 2)

Fig. 4

Chemical structures of PP2A inhibitors—okadaic acid, fostriecin and rubratoxin A

Fig. 5

Chemical structures of calyculin A, phosphocalyculin A, hemicalyculin A and motuporin. For the structure of okadaic acid, see Fig. 4