Biochemical characterization of a Dictyostelium myosin II heavy‐chain phosphatase that promotes filament assembly

Abstract

In Dictyostelium cells, myosin II is found as cytosolic nonassembled monomers and cytoskeletal bipolar filaments. It is thought that the phosphorylation state of three threonine residues in the tail of myosin II heavy chain regulates the molecular motor's assembly state and localization. Phosphorylation of the myosin heavy chain at threonine residues 1823, 1833 and 2029 is responsible for maintaining myosin in the nonassembled state, and subsequent dephosphorylation of these residues is a prerequisite for assembly into the cytoskeleton. We report here the characterization of myosin heavy-chain phosphatase activities in Dictyostelium utilizing myosin II phosphorylated by myosin heavy-chain kinase A as a substrate. One of the myosin heavy-chain phosphatase activities was identified as protein phosphatase 2A and the purified holoenzyme was composed of a 37-kDa catalytic subunit, a 65-kDa A subunit and a 55-kDa B subunit. The protein phosphatase 2A holoenzyme displays two orders of magnitude higher activity towards myosin phosphorylated on the heavy chains than it does towards myosin phosphorylated on the regulatory light chains, consistent with a role in the control of filament assembly. The purified myosin heavy-chain phosphatase activity promotes bipolar filament assembly in vitro via dephosphorylation of the myosin heavy chain. This system should provide a valuable model for studying the regulation and localization of protein phosphatase 2A in the context of cytoskeletal reorganization.Keywords: Dictyostelium; filament assembly; myosin II; phosphatase.In Dictyostelium cells, the conventional myosin, myosin II, is required for cytokinesis, cell-surface receptor capping, cortical tension maintenance, and multicellular development based upon the phenotypic defects observed in myosin null cells and upon other cellular analysis [1±6]. Myosin II function in vivo requires assembly into bipolar filaments, which can interact with the cortical actin cytoskeleton. Phosphorylation of the myosin heavy chain (MHC) plays a critical role in the regulation of filament assembly and localization of myosin in this system [7].Phosphorylation of three threonine residues at positions 1823, 1833, and 2029, located in the C-terminus of the myosin II coiled-coil heavy-chain tail, by the 130-kDa myosin heavy-chain kinase A (MHCK A) inhibits filament selfassembly in vitro and in vivo [8±10]. The inhibition of myosin II filament assembly by phosphorylation may be due to bending of the myosin tail which would sterically hinder selfassembly [11]. The functional importance of phosphorylation of these threonine sites has been demonstrated by expressing mutated forms of myosin II in Dictyostelium myosin II null cells [9]. In these studies, the three threonine residues were converted into alanine (3Â Ala) to abolish phosphorylation at these sites or into aspartate (3Â Asp) to mimic the phosphothreonine sites. It was observed that in vitro the 3Â Ala myosin formed filaments but the 3Â Asp myosin failed to. In addition, in vivo, myosin from 3Â Ala-...

WD Repeat Domains Target Dictyostelium Myosin Heavy Chain Kinases by Binding Directly to Myosin Filaments

“…Myosin was purified by a modification of the method of AguadoVelasco, et al (20), as has been described (11). Briefly, an initial acto-myosin precipitate was subjected to several assembly/disassembly cycles followed by a final gel filtration step to remove residual actin.…”

“…MHC phosphorylation appears to be regulated in Dictyostelium through the combined activities of at least one MHC phosphatase (11) and a group of MHC kinases (2,12). Dictyostelium MHC kinase (MHCK) A was the first of the heavy chain kinases to be purified.…”

Myosin heavy chain kinases play essential roles in Ca2+, but not cAMP, chemotaxis and the natural aggregation of Dictyostelium discoideum

“…The defects in the 3XASP and 3XALA mutants indicate that the phosphorylationdephosphorylation cycle plays a role both in basic cell motility and chemotaxis. This cycle is regulated by myosin heavy chain kinases and phosphatases (Murphy and Egelhoff, 1999;Rai and Egelhoff, 2011), which one would assume are the targets of signal transduction pathways that coordinate remodeling of the cortical cytoskeleton during basic cell motility and chemotaxis.…”

Role of B Regulatory Subunits of Protein Phosphatase Type 2A in Myosin II Assembly Control in Dictyostelium discoideum

“…Murphy and Egelhoff reported PP2A as a major MHC phosphatase in Dictyostelium in studies using purified myosin II and biochemical fractionation approaches (16). In those studies, PP2A holoenzyme displayed substantially higher activity toward myosin phosphorylated on the heavy chains than it did toward myosin phosphorylated on the regulatory light chains (RLC), supporting the hypothesis that PP2A may have a physiological role in the control of filament assembly (16).…”

“…The first of these, here designated B55␣ (phr2aB␣), was originally identified via mass spectroscopy and Western blotting as a subunit of the biochemically isolated trimeric MHC phosphatase described previously (16,17). In the current work, we describe the consequences of gene disruption of phr2aB␣ and consequences of gene disruption of a closely related gene in the Dictyostelium genome that has been annotated phr2aB␤.…”