Regulation of non-muscle myosin assembly by calmodulin-dependent light chain kinase

The presence of actin and myosin in non-muscle cells suggests that they may be involved in a wide range of cellular contractile activities. The generally accepted view is that interaction between actin and myosin in these cells and in vertebrate smooth muscle, is regulated by the level of phosphorylation of the 20,000-molecular weight (MW) light chain. In the absence of calcium, this light chain is not phosphorylated and the myosin cannot interact with actin. Calcium activates a specific calmodulin-dependent kinase which phosphorylates the light chain, initiating actin-myosin interaction. Although most studies on the role of phosphorylation have concentration on the regulation of actin-activated myosin Mg-ATPase activity, phosphorylation of the light chain also seems to control the assembly of smooth muscle myosin into filaments. Using purified smooth muscle light chain kinase, we have confirmed this observation. We report here studies of myosins isolated from the two non-muscle sources, thymus cells and platelets. We observed that these myosins are assembled into filaments at physiological ionic strength and Mg-ATP concentrations, only when the 20,000-MW light chain is phosphorylated.     

The essential light chains constitute part of the active site of smooth muscle myosin

Myosin, a major contractile protein, characteristically possesses a long coiled-coil alpha-helical tail and two heads. Each head contains both an actin binding site and an ATPase site and is formed from the NH2-terminal half of one of the two heavy chains (relative molecular mass, Mr, 200,000) and a pair of light chains; the so-called regulatory and essential light chains of approximately Mr 20,000 each. Recently we have identified Trp 130 of the myosin heavy chain from rabbit skeletal muscle as an active-site amino-acid residue after labelling with a new photoaffinity analogue of ADP, N-(4-azido-2-nitrophenyl)-2-aminoethyl diphosphate (NANDP). Nonspecific labelling was eliminated by first trapping NANDP at the active site with thiol crosslinking agents. Exclusive labelling of the heavy chains with no labelling of the light chains agreed with previous findings that the heavy chains alone contain the actin-activated Mg-ATPase activity of rabbit skeletal myosin. Here we report similar photolabelling experiments with smooth muscle myosin (chicken gizzard) in which 3H-NANDP is trapped at the active site with vanadate and which show that both the heavy chains and the essential light chains are labelled. The results indicate that both chains contribute to the ATP binding site and represent the first direct evidence for participation of the essential light chains in the active site of any type of myosin.     

5DFRXXL region of long myosin light chain kinase causes F-actin bundle formation

Long myosin light chain kinase （L-MLCK） contains five DFRXXL motifs with ability to bind F-actin. Binding stoichiometry data indicated that each DFRXXL motif might bind each G-actin, but its biological significance remained unknown. We hypothesized that L-MLCK might act as an F-actin bundle peptides by its multiple binding sites of 5DFRXXL motifs to actin. In order to characterize F-actin-bundle formation properties of 5DFRXXL region of long myosin light chain kinase, we expressed and purified 5DFRXXL peptides tagged with HA in vitro. The properties of 5DFRXXL peptides binding to myofilaments or F-actin were analyzed by binding stoichiometries assays. The results indicated that 5DFRXXL peptides bound to myofilaments or F-actin with high affinity. KD values of 5DFRXXL binding to myofilaments and F-actin were 0.45 and 0.41 μmol/L, respectively. Cross-linking assay demonstrated that 5DFRXXL peptides could bundle F-actin efficiently. Typical F-actin bundles were observed morphologically through determination of confocal and electron microscopy after adding 5DFRXXL peptides. After transfection of pEGFP-5DFRXXL plasmid into eukaryocyte, spike structure was observed around cell membrane edge. We guess that such structure formation may be attributable to F-actin over-bundle formation caused by 5DFRXXL peptides. Therefore, we suppose that L-MLCK may be a new bundling protein and somehow play a certain role in organization of cell skeleton besides mediating cell contraction by it kinase activity.     

人心室肌球蛋白轻链1基因的克隆及其体外的表达与纯化

通过聚合酶链式反应方法扩增人心室肌球蛋白轻链1的基因片段,并将其克隆到pET-22b质粒,构建表达重组体,转化大肠杆菌BL21细胞,转化子细胞经IPTG诱导,目的蛋白在大肠杆菌中得到高效表达,凝胶成像扫描检测,其表达量接近细胞总蛋白的40%,经Ni2+亲和层析使目的蛋白得到纯化。     

Probing the calcium-induced conformational transition of troponin C with site-directed mutants

The contraction of skeletal muscle is regulated by calcium binding to troponin C (TnC). TnC consists of two spatially independent domains, each of which contains two metal ion binding sites. Calcium binding to the regulatory sites of the N-terminal domain triggers muscle contraction by a series of conformational changes. Site-directed mutagenesis offers a means of elucidating the links in this signal path between TnC and actin-myosin crossbridges. Such mapping is possible if the mutants shift the equilibrium between 'on' and 'off' states of the regulatory complex while maintaining the coupling between calcium binding and tension development. Candidate amino-acid residues for yielding this information would be in positions remote from the calcium-binding sites and from the site of development of tension. Analysis of the crystal structure of TnC and of the model of the calcium-activated molecule has enabled us to identify two such residues: Glu 57 and Glu 88. In separate experiments we have replaced each of these residues by lysines. The resulting reduction in calcium affinity indicates that these residues have a long-range effect on calcium binding. This result may reflect the formation of a salt bridge between positions 57 and 88 that is not present in the native molecule. Moreover, the level of tension recovery when the mutants are incorporated into muscle suggests that the interaction between TnC and other muscle components has also been altered. Thus, these residues may participate in the contraction signal transmission.     

Control of tension development in scallop muscle fibres with foreign regulatory light chains

We have shown previously that chemically skinned fibre bundles from scallop muscle can be desensitized to the action of calcium by the removal of the regulatory light chains of myosin and that sensitivity can be restored by the re-addition of scallop light chains. We have now confirmed these results and extended our observations to fibre bundles from which one or both of the regulatory light chains per myosin have been removed (by treatment with EDTA at 7 and 25 degrees C, respectively) and replaced by the corresponding light chains from other species. In the case of a double substitution--where both light chains were replaced--sensitivity was restored completely by light chains from other molluscs and from gizzard muscle; for a single substitution there was restoration of sensitivity by all the light chains tested.     

Clathrin light chains contain brain-specific insertion sequences and a region of homology with intermediate filaments

The primary structures of four bovine clathrin light chains have been determined. Light chains LCa and LCb are homologous proteins encoded by different genes. In the brain the messenger RNA from these genes undergoes differential splicing to yield proteins having centrally inserted brain-specific sequences. A potentially alpha-helical region of the clathrin light chains shows homology with intermediate filament proteins.