1) Last year, the heavy chains of NM2A, NM2B and NM2C with N-terminal FLAG tags were separately co-expressed in Sf9 cells with the regulatory and essential light chains (RLC and ELC). The proteins were purified to electrophoretic homogeneity by affinity chromatography. The minimal concentrations required for polymerization were determined by light scattering as a function of myosin concentration after polymerization overnight at 0 in 10 mM MOPS, pH 7.0, 150 mM NaCl, 0.1 mM EGTA, 2 mM MgCl2, with and without addition of 1 mM ATP, and before and after phosphorylation of the RLC. The minimal protein concentration required for polymerization of all three NM2s, both RLC-phosphorylated and RLC-unphosphorylated, were higher in the presence of ATP than in its absence, but the differences were very much less than expected from previous published results for NM2s purified from thymus and intestinal brush border. 1) Last year, the heavy chains of NM2A, NM2B and NM2C with N-terminal FLAG tags were separately co-expressed in Sf9 cells with the regulatory and essential light chains (RLC and ELC). The proteins were purified to electrophoretic homogeneity by affinity chromatography. The minimal concentrations required for polymerization were determined by measuring light scattering as a function of myosin concentration after polymerization overnight at 0 in 10 mM MOPS, pH 7.0, 150 mM NaCl, 0.1 mM EGTA, 2 mM MgCl2, with and without addition of 1 mM ATP, and before and after phosphorylation of the RLC. The minimal protein concentration required for polymerization of all three NM2s, both RLC-phosphorylated and RLC-unphosphorylated, were higher in the presence of ATP than in its absence, but the differences were very much less than expected from previously published results for NM2s purified from thymus and intestinal brush border. This year, we confirmed previous reports that addition of 1 mM ATP substantially reduces the light scattering of polymerized RLC-unphosphorylated NM2s, and that this is reversed by phosphorylation of the RLC by myosin light chain kinase. It has been proposed that these changes result from substantial depolymerization of unphosphorylated NM2 filaments upon addition of ATP and filament repolymerization upon RLC-phosphorylation. However, the difference in the extent of polymerization of unphosphorylated and RLC-phosphorylated NM2s in the presence of ATP is much too small to explain the observed changes in light scattering. We found that the changes in light scattering are due to previously unknown differences in light scattering of equal concentrations of polymerized unphosphorylated NM2s in the presence and absence of ATP. This decrease in light scattering of polymerized RLC-unphosphorylated NM2 filaments upon addition of ATP results from a previously undescribed ATP-induced decrease in filament size, and the formation of tetrameric and hexameric myosin oligomers. ATP has little, if any, effect on the size of filaments of RLC-phosphorylated NM2s, which is similar to that of unphosphorylated filaments in the absence of ATP. 2) There is multiple evidence for the functional association of NM2s with biological membranes with evidence both for and against the direct association of NM2s with the membrane lipids. There are minimal, but positive, data for association of NM2s with liposomes, but there are no data using pure NM2A, NM2B and NM2C, and no data on the molecular basis of the binding of these myosins to phospholipids. We found that pure, recombinant full length NMIIA, NMIIB and NMIIC bind to 100% phosphatidylserine (PS) liposomes but not to 100% phosphatidylcholine (PC) liposomes. We find that binding to PC liposomes containing various concentrations of either PS, phosphatidylinositol-4,5-diphosphate (PIP2), or phosphatidylinositol-3,4,5-triphosphate (PIP3) is proportional to the net negative charge of the liposomes with no specificity for PIP2, PIP3 or PS. Contrary to previous proposals, from studies of the association of myosin rods with liposomes, we find that deletion of the non-helical tailpiece does not affect binding of the three recombinant NM2s to liposomes. A paper reporting these and related results has been submitted.
| Liu, Xiong; Shu, Shi; Yu, Shuhua et al. (2014) Biochemical and biological properties of cortexillin III, a component of Dictyostelium DGAP1-cortexillin complexes. Mol Biol Cell 25:2026-38 |
| Heissler, Sarah M; Liu, Xiong; Korn, Edward D et al. (2013) Kinetic Characterization of the ATPase and Actin-activated ATPase Activities of Acanthamoeba castellanii Myosin-2. J Biol Chem 288:26709-20 |
| Liu, Xiong; Hong, Myoung-Soon; Shu, Shi et al. (2013) Regulation of the filament structure and assembly of Acanthamoeba myosin II by phosphorylation of serines in the heavy-chain nonhelical tailpiece. Proc Natl Acad Sci U S A 110:E33-40 |
| Liu, Xiong; Lee, Duck-Yeon; Cai, Shutao et al. (2013) Regulation of the actin-activated MgATPase activity of Acanthamoeba myosin II by phosphorylation of serine 639 in motor domain loop 2. Proc Natl Acad Sci U S A 110:E23-32 |
