Inhibition of Conformational Change in Smooth Muscle Myosin by a Monoclonal Antibody against the 17-kDa Light Chain

Development of novel anti-CD19 antibody-drug conjugates for B-cell lymphoma treatment

B-cell lymphoma remains one of the most refractory tumors, and as such the development of novel treatment approaches, such as antibody-drug conjugates (ADCs), is required. To improve the stability and homogeneity of the ADCs, a humanized anti-CD19 monoclonal antibody (RC58) was developed in the present study. RC58 was based on the CD19 antigen as a potential molecular target of human B-cell lymphomas. RC58 has high CD19-binding affinity and can be internalized in CD19-positive cells through endocytosis. Furthermore, three types of RC58-based ADCs (ADC-1, ADC-2, and ADC-3) were generated using three kinds of Maleimide-PEG-based linkers with two different cytotoxins. The anti-tumor activities of the ADCs were confirmed by in vitro and in vivo experiments. The stability of the ADCs was also evaluated by incubation in human plasma for 10 days. In vitro experiments showed that the three ADCs had distinct inhibitory effects on several B-lymphoma cell lines. Meanwhile, a close correlation between efficacy and drug concentration was found in a nude mouse xenograft model of human B-cell lymphoma, after treatment with RC58-based ADCs. Our results suggest that ADC-1, with high efficiency, could be used as a potential therapeutic agent for human B-cell malignancies.

KIF5A transports collagen vesicles of myofibroblasts during pleural fibrosis

Fibrosis involves the production of extracellular matrix proteins in tissues and is often preceded by injury or trauma. In pleural fibrosis excess collagen deposition results in pleural thickening, increased stiffness and impaired lung function. Myofibroblasts are responsible for increased collagen deposition, however the molecular mechanism of transportation of procollagen containing vesicles for secretion is unknown. Here, we studied the role of kinesin on collagen-1 (Col-1) containing vesicle transportation in human pleural mesothelial cells (HPMCs). Among a number of cargo transporting kinesins, KIF5A was notably upregulated during TGF-β induced mesothelial-mesenchymal transition (MesoMT). Using superresolution structured illumination microscopy and the DUO-Link technique, we found that KIF5A colocalized with Col-1 containing vesicles. KIF5A knock-down significantly reduced Col-1 secretion and attenuated TGF-β induced increment in Col-1 localization at cell peripheries. Live cell imaging revealed that GFP-KIF5A and mCherry-Col-1 containing vesicles moved together. Kymography showed that these molecules continuously move with a mean velocity of 0.56 μm/sec, suggesting that the movement is directional but not diffusion limited process. Moreover, KIF5A was notably upregulated along with Col-1 and α-smooth muscle actin in pleural thickening in the carbon-black bleomycin mouse model. These results support our hypothesis that KIF5A is responsible for collagen transportation and secretion from HPMCs.

A novel regulatory mechanism of myosin light chain phosphorylation via binding of 14-3-3 to myosin phosphatase

Myosin II phosphorylation-dependent cell motile events are regulated by myosin light-chain (MLC) kinase and MLC phosphatase (MLCP). Recent studies have revealed myosin phosphatase targeting subunit (MYPT1), a myosin-binding subunit of MLCP, plays a critical role in MLCP regulation. Here we report the new regulatory mechanism of MLCP via the interaction between 14-3-3 and MYPT1. The binding of 14-3-3beta to MYPT1 diminished the direct binding between MYPT1 and myosin II, and 14-3-3beta overexpression abolished MYPT1 localization at stress fiber. Furthermore, 14-3-3beta inhibited MLCP holoenzyme activity via the interaction with MYPT1. Consistently, 14-3-3beta overexpression increased myosin II phosphorylation in cells. We found that MYPT1 phosphorylation at Ser472 was critical for the binding to 14-3-3. Epidermal growth factor (EGF) stimulation increased both Ser472 phosphorylation and the binding of MYPT1-14-3-3. Rho-kinase inhibitor inhibited the EGF-induced Ser472 phosphorylation and the binding of MYPT1-14-3-3. Rho-kinase specific siRNA also decreased EGF-induced Ser472 phosphorylation correlated with the decrease in MLC phosphorylation. The present study revealed a new RhoA/Rho-kinase-dependent regulatory mechanism of myosin II phosphorylation by 14-3-3 that dissociates MLCP from myosin II and attenuates MLCP activity.

M20, the small subunit of PP1M, binds to microtubules

Myosin light chain phosphatase (PP1M) is composed of three subunits, i.e., M20, MBS, and a catalytic subunit. Whereas MBS is assigned as a myosin binding subunit, the function of M20 is unknown. In the present study, we found that M20 binds to microtubules. The binding activity was revealed by cosedimentation of M20 with microtubules and binding of tubulin to M20 affinity resin. Green fluorescent protein (GFP)-tagged M20 (M20-GFP) was expressed in chicken primary smooth muscle cells and COS-7 cells and was used as a probe for studying the association between M20 and microtubules in living cells. M20-GFP was localized on filamentous structures in both cell types. Colocalization analysis revealed that M20-GFP colocalized with tubulin. Treatment with nocodazole, but not cytochalasin B, abolished the filamentous structure of M20-GFP. These results indicate that M20-GFP associates with microtubules in cells. Microinjection of rhodamine-tubulin into the M20-expressing cells revealed that incorporation of rhodamine-tubulin into microtubules was significantly facilitated by microtubule-associated M20. Consistent with this result, M20 enhanced the rate of tubulin polymerization in vitro and produced elongated microtubules. These results suggest that M20 has a microtubule binding activity and plays a role in regulating microtubule dynamics.

Translocation of telokin by cGMP signaling in smooth muscle cells

Telokin is an acidic protein with a sequence identical to the COOH-terminal domain of myosin light chain kinase (MLCK) produced by an alternate promoter of the MLCK gene. Although it is abundantly expressed in smooth muscle, its physiological function is not understood. In the present study, we attempted to clarify the function of telokin by analyzing its spatial and temporal localization in living single smooth muscle cells. Primary cultured smooth muscle cells were transfected with green fluorescent protein (GFP)-tagged telokin. The telokin-GFP localized mostly diffusely in cytosol. Stimulation with both sodium nitroprusside (SNP) and 8-bromo-cyclic GMP induced translocation of GFP-tagged telokin to near plasma membrane in living single smooth muscle cells. The translocation was slow, and it took more than 10 min at room temperature. Mutation of the phosphorylation sites of telokin (S13A, S19A, and S13A/S19A) significantly attenuated SNP-induced translocation. Both KT-5823 (cGMP-dependent protein kinase inhibitor) and PD-98059 (mitogen-activated protein kinase inhibitor) diminished the telokin-GFP translocation. These results suggest that telokin changes its intracellular localization because of phosphorylation at Ser13 and/or Ser19 via the cGMP-signaling pathway.

DOC-2/DAB2 is the binding partner of myosin VI

Myosin VI is a molecular motor that moves processively along actin filaments and is believed to play a role in cargo movement in cells. Here we found that DOC-2/DAB2, a signaling molecule inhibiting the Ras cascade, binds to myosin VI at the globular tail domain. DOC-2/DAB2 binds stoichiometrically to myosin VI with one molecule per one myosin VI heavy chain. The C-terminal 122 amino acid residues of DOC-2/DAB2, containing the Grb2 binding site, is identified to be critical for the binding to myosin VI. Actin gliding assay revealed that the binding of DOC-2/DAB2 to myosin VI can support the actin filament gliding by myosin VI, suggesting that it can function as a myosin VI anchoring molecule. The C-terminal domain but not the N-terminal domain of DOC-2/DAB2 functions as a myosin VI anchoring site. The present findings suggest that myosin VI plays a role in transporting DOC-2/DAB2, a Ras cascade signaling molecule, thus involved in Ras signaling pathways.

Purification and identification of a 42-kilodalton abscisic acid-specific-binding protein from epidermis of broad bean leaves

Purification of abscisic acid (ABA)-binding proteins is considered to constitute a major step toward isolating ABA receptors. We report here that an ABA-binding protein was for the first time, to our knowledge, purified from the epidermis of broad bean (Vicia faba) leaves via affinity chromatography. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis, isoelectric focusing electrophoresis, and isoelectric focusing/sodium dodecyl sulfate-polyacrylamide gel electrophoresis two-dimensional electrophoresis of the purified ABA-binding protein all identified a single protein band with a molecular mass of 42 kD and an isoelectric point 4.86. The Scatchard plot for the purified protein showed a linear function with a maximum binding activity of 0.87 mol mol(-1) protein and an equilibrium dissociation constant of 21 nM, indicating that the purified protein may be a monomeric one, possessing one binding site. The ABA-binding protein was enriched more than 300-fold with a yield of 14%. (-)ABA and trans-ABA were substantially incapable of displacing (3)H-(+/-)ABA bound to the ABA-binding protein, and (+/-)ABA was less effective than (+)ABA in the competition. These findings allow establishment of the stereospecificity of the 42-kD protein and suggest its ABA receptor nature. Pretreatment of the guard cell protoplasts of broad bean leaves with the monoclonal antibody raised against the 42-kD protein significantly decreased the ABA specific-induced phospholipase D activity in a dose-dependent manner. This physiological significance provides more clear evidence for the potential ABA-receptor nature of the 42-kD protein.

Dual regulation of mammalian myosin VI motor function

Myosin VI is expressed in a variety of cell types and is thought to play a role in membrane trafficking and endocytosis, yet its motor function and regulation are not understood. The present study clarified mammalian myosin VI motor function and regulation at a molecular level. Myosin VI ATPase activity was highly activated by actin with K(actin) of 9 microm. A predominant amount of myosin VI bound to actin in the presence of ATP unlike conventional myosins. K(ATP) was much higher than those of other known myosins, suggesting that myosin VI has a weak affinity or slow binding for ATP. On the other hand, ADP markedly inhibited the actin-activated ATPase activity, suggesting a high affinity for ADP. These results suggested that myosin VI is predominantly in a strong actin binding state during the ATPase cycle. p21-activated kinase 3 phosphorylated myosin VI, and the site was identified as Thr(406). The phosphorylation of myosin VI significantly facilitated the actin-translocating activity of myosin VI. On the other hand, Ca(2+) diminished the actin-translocating activity of myosin VI although the actin-activated ATPase activity was not affected by Ca(2+). Calmodulin was not dissociated from the heavy chain at high Ca(2+), suggesting that a conformational change of calmodulin upon Ca(2+) binding, but not its physical dissociation, determines the inhibition of the motility activity. The present results revealed the dual regulation of myosin VI by phosphorylation and Ca(2+) binding to calmodulin light chain.

The tip of the coiled-coil rod determines the filament formation of smooth muscle and nonmuscle myosin

Myosin II self-assembles to form thick filaments that are attributed to its long coiled-coil tail domain. The present study has determined a region critical for filament formation of vertebrate smooth muscle and nonmuscle myosin II. A monoclonal antibody recognizing the 28 residues from the C-terminal end of the coiled-coil domain of smooth muscle myosin II completely inhibited filament formation, whereas other antibodies recognizing other parts of the coiled-coil did not. To determine the importance of this region in the filament assembly in vivo, green fluorescent protein (GFP)-tagged smooth muscle myosin was expressed in COS-7 cells, and the filamentous localization of the GFP signal was monitored by fluorescence microscopy. Wild type GFP-tagged smooth muscle myosin colocalized with F-actin during interphase and was also recruited into the contractile ring during cytokinesis. Myosin with the nonhelical tail piece deleted showed similar behavior, whereas deletion of the 28 residues at the C-terminal end of the coiled-coil domain abolished this localization. Deletion of the corresponding region of GFP-tagged nonmuscle myosin IIA also abolished this localization. We conclude that the C-terminal end of the coiled-coil domain, but not the nonhelical tail piece, of myosin II is critical for myosin filament formation both in vitro and in vivo.

Zipper-interacting protein kinase induces Ca(2+)-free smooth muscle contraction via myosin light chain phosphorylation

The inhibition of myosin phosphatase evokes smooth muscle contraction in the absence of Ca(2+), yet the underlying mechanisms are not understood. To this end, we have cloned smooth muscle zipper-interacting protein (ZIP) kinase cDNA. ZIP kinase is present in various smooth muscle tissues including arteries. Triton X-100 skinning did not diminish ZIP kinase content, suggesting that ZIP kinase associates with the filamentous component in smooth muscle. Smooth muscle ZIP kinase phosphorylated smooth muscle myosin as well as the isolated 20-kDa myosin light chain in a Ca(2+)/calmodulin-independent manner. ZIP kinase phosphorylated myosin light chain at both Ser(19) and Thr(18) residues with the same rate constant. The actin-activated ATPase activity of myosin increased significantly following ZIP kinase-induced phosphorylation. Introduction of ZIP kinase into Triton X-100-permeabilized rabbit mesenteric artery provoked a Ca(2+)-free contraction. A protein phosphatase inhibitor, microcystin LR, also induced contraction in the absence of Ca(2+), which was accompanied by an increase in both mono- and diphosphorylation of myosin light chain. The observed sensitivity of the microcystin-induced contraction to various protein kinase inhibitors was identical to the sensitivity of isolated ZIP kinase to these inhibitors. These results suggest that ZIP kinase is responsible for Ca(2+) independent myosin phosphorylation and contraction in smooth muscle.

Effects of the regulatory light chain phosphorylation of myosin II on mitosis and cytokinesis of mammalian cells

Myosin plays an important role in mitosis, especially during cytokinesis. Although it has been assumed that phosphorylation of regulatory light chain of myosin (RLC) controls motility of mammalian non-muscle cells, the functional significance of RLC phosphorylation remains uninvestigated. To address this problem, we have produced unphosphorylatable RLC (T18A/S19A RLC) and overexpressed it in COS-7 cells and normal rat kidney cells. Overexpression of T18A/S19A RLC but not wild type RLC almost completely abolished concanavalin A-induced receptor cap formation. The results indicate that myosin phosphorylation is critical for concanavalin A-induced gathering of surface receptors. T18A/S19A RLC overexpression resulted in the production of multinucleated cells, suggesting the failure of proper cell division in these cells. Video microscopic observation revealed that cells expressing T18A/S19A RLC showed abnormalities during mitosis in two respects. One is that the cells produced abnormal cleavage furrows, resulting in incomplete cytokinesis, which suggests that myosin phosphorylation is important for the normal recruitment of myosin molecules into the contractile ring structure. The other is that separation of chromosomes from the metaphase plate is disrupted in T18A/S19A RLC expressing cells, thus preventing proper transition from metaphase to anaphase. These results suggest that, in addition to cytokinesis, myosin and myosin phosphorylation play a role in the karyokinetic process.

Effects of mutations in the gamma-phosphate binding site of myosin on its motor function

The role of the highly conserved residues in the gamma-phosphate binding site of myosin upon myosin motor function was studied. Each of five residues (Ser181, Lys185, Asn235, Ser236, and Arg238) in smooth muscle myosin was mutated. K185Q has neither a steady state ATPase nor an initial Pi burst. Although ATP and actin bind to K185Q, it is not dissociated from actin by ATP. These results indicate that the hydrolysis of bound ATP by K185Q is inhibited. S236T has nearly normal basal Mg2+-ATPase activity, initial Pi burst, ATP-induced enhancement of intrinsic tryptophan fluorescence, and ATP-induced dissociation from actin. However, the actin activation of the Mg2+-ATPase activity and actin translocation of S236T were blocked. In contrast S236A has nearly normal enzymatic properties and actin-translocating activity. These results indicate that 1) the hydroxyl group of Ser236 is not critical as an intermediary of proton transfer during the ATP hydrolysis step, and 2) the bulk of the extra methyl group of the threonine residue in S236T blocks the acceleration of product release from the active site by actin. Arg238, which interacts with Glu459 at the Switch II region, was mutated to Lys and Ile, respectively. R238K has essentially normal enzymatic activity and motility. In contrast, R238I does not hydrolyze ATP or support motility, although it still binds ATP. These results indicate that the charge interaction between Glu459 and Arg238 is critical for ATP hydrolysis by myosin. Other mutants, S181A, S181T, and N235I, showed nearly normal enzymatic and motile activity.

Requirement of the two-headed structure for the phosphorylation dependent regulation of smooth muscle myosin

It is known for smooth muscle myosin that while acto-HMM ATPase activity is regulated by phosphorylation, acto-S-1 ATPase activity is not regulated. To clarify the heavy chain structure required for the regulation, smooth muscle myosin containing 7 different lengths of the S-2 portion were expressed in Sf9 insect cells using Baculovirus expression system. Myosin containing longer than 991 residues of heavy chain formed a stable two-headed structure while myosin with shorter than 944 residues of heavy chain formed a single-headed structure, indicating that the residues Gln945-Asp991 are critical for the formation of the two-headed structure. The actin activated ATPase activity of myosin mutants having a two-headed structure was activated by phosphorylation while that of myosin mutants that failed to form the two-headed structure was completely independent of phosphorylation. These results suggest that the two-headed structure is critical for the phosphorylation-dependent regulation.

Inhibition of 20-kDa myosin light chain exchange by monoclonal antibodies against 17-kDa myosin light chain

Two anti-17,000 Da myosin light chain (LC17) monoclonal antibodies (MM2 and MM10), which increase the actin-activated Mg(2+)-ATPase activity of dephosphorylated smooth muscle myosin, inhibited the exchange of the 20,000 Da regulatory light chain of myosin (LC20). MM2, which shows higher potency of activation of ATPase activity, inhibited the exchange more extensively than MM10, suggesting that there is a correlation between the activation of ATPase activity and the inhibition of the LC20 exchange. The inhibition of the exchange was observed for intact myosin and heavy meromyosin but not subfragment 1, suggesting that the heavy chain at the head-rod junction is involved in the inhibition of LC20 exchange by anti-LC17 antibodies. Alternatively, the interaction between the two heads of the myosin molecule may influence the inhibition of LC20 exchange. These results suggest that LC20 interacts with both LC17 and the heavy chain, and the interaction between LC20 and LC17 is involved in the activation of actin-activated ATPase activity of smooth muscle myosin.

Mutagenesis of the phosphorylation site (serine 19) of smooth muscle myosin regulatory light chain and its effects on the properties of myosin

A full-length cDNA of smooth muscle regulatory light chain was obtained and the recombinant regulatory light chain was expressed in an Escherichia coli expression system. The recombinant regulatory light chain was introduced into myosin or HMM using a subunit exchange strategy [Morita, J., Takashi, R., & Ikebe, M. (1991) Biochemistry 30, 9539-9545]. The recombinant wild-type regulatory light chain exhibited the same biological properties as the natural isolate, i.e., phosphorylation at Ser-19 by myosin light-chain kinase and phosphorylation-activated actomyosin ATPase activity. To clarify whether or not the activation of the ATPase by phosphorylation is simply due to the introduction of negative charge, we produced three mutant light chains. Two of them contain Ser-19 substituted by either Asp or Ala and the third contains Asp substituted for both Thr-18 and Ser-19. Incorporation of the Asp mutant partially activated actomyosin ATPase activity but the activation level was significantly lower than that by phosphorylation. The Asp/Asp mutant further activated actomyosin ATPase activity. On the other hand, the Ala mutant did not affect the ATPase activity. Incorporation of Asp mutant slightly affected the 10S-6S conformational transition and filament formation of myosin. The Asp/Asp mutant more significantly affected the 10S-6S conformational transition and filament formation of myosin. These results suggested that the activation of smooth muscle myosin requires the introduction of negative charge in the defined spacial position. Using Ser-19 deficient mutants, the effects of Thr-18 phosphorylation on myosin function was also studied. Actin-activated ATPase activity of myosin was significantly activated by phosphorylation of Thr-18.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional effects of LC1-reassociation with cardiac papain Mg.S1

The effect of LC1 on cardiac myosin structure and activity was investigated using as a model S1 prepared by papain digestion in the presence of Mg2+. The resulting S1 contained LC2 but a part of the N-terminal region of LC1 was cleaved. Sequencing the N-terminal part of the band migrating below LC1 on SDS gels revealed it to consist of alternating alanyl and prolyl residues thus establishing LC1 as the origin of this band. However, Western blots did not reveal any LC1 while radioimmunoassays indicated it to be present at the 5% level suggesting the anti-LC1 antibody used in these experiments did not recognize the C-terminal portion of LC1 still attached to Mg.S1. Mixing a 10-15 M excess of isolated light chains with Mg.S1 in the presence of 10 mM ATP, 12 mM MgCl2, 4.7 M NH4Cl allowed LC1 to recombine with LC1-deficient Mg.S1. Equilibrium ultracentrifugation analysis revealed a highly heterogeneous LC1-deficient S1 which upon recombination with intact LC1 became monodisperse as indicated by the superimposition of molecular weight averages all across the centrifuge cell. LC1-deficient Mg.S1 had a Vm of 0.4 s-1, Ka of 30 microM and a Kbind of 28 microM. In the presence of intact LC1, Vm rose to 0.8 s-1 while Ka and Kbind were reduced to 7.5 and 12 microM, respectively. The fourfold decrease in Ka strongly indicated an increased affinity for actin by Mg.S1 in the presence of uncleaved LC1. Also, Ca(2+)-regulation of dog heart myofibrils was suppressed when Ca(2+)-activated MgATPase assays, as a function of Ca2+, were performed in the presence of anti-LC1 antibodies. These observations suggest the presence of intact, uncleaved LC1 in S1 is required for the stability of S1 heavy chains and proper Ca(2+)-regulation.

Smooth muscle myosin subfragment-1 is a kinetic analogue for heavy meromyosin in the extended conformation

The 10S-->6S (Flexed-->Extended) transition in smooth muscle myosin is related to increased ATPase activity, but there is controversy over whether the analogous 9S-->7S transition in HMM is also associated with ATPase activity. We therefore studied the association of ionic strength, phosphorylation, and ATPase activity for HMM as compared to S1 which has no apparent flexed conformation. In addition, we performed both steady state and single turnover analyses, to control for artifacts due to multiple subfragment populations that might skew steady state results. At low ionic strength where myosin and HMM are in the flexed conformation, HMM had a near zero ATPase activity while S-1 had a high ATPase rate (0.07 s-1). At 400 mM ionic strength, where both myosin and HMM are in the extended conformation, S1 and HMM had the same ATPase rate (0.04 s-1). Phosphorylation did not affect S1 significantly, but shifted the HMM curve to higher rates at lower ionic strengths. Both steady state and single turnover experiments gave the same results, indicating that steady state results were not skewed by multiple subfragment populations. These data indicate that HMM has a conformation-ATPase relation similar to that observed with myosin. Furthermore, these findings suggest that the S1 ATPase rate corresponds to that of HMM in the extended conformation.

Structural changes induced in scallop heavy meromyosin molecules by Ca2+ and ATP

We have used physicochemical and ultrastructural methods to investigate the effects of Ca2+ and ATP on the structure of purified heavy meromyosin (HMM) from the striated adductor muscle of the scallop, a species with myosin-linked regulation. Using papain as a structural probe, we found that, in the presence of ATP, the head/tail junction was five times more susceptible to digestion at high levels of Ca2+ than at low levels. By HPLC gel filtration, two fractions of scallop HMM with different Stokes radii were detected in the presence of ATP at low Ca2+, while at high Ca2+ a single peak with the larger Stokes radius predominated. Electron microscopy of rotary-shadowed HMM suggested that molecules with the smaller Stokes radius had their heads bent back towards their tails, while those with the larger radius had heads pointing away from the tail. The number of molecules with their heads bent back decreased at high Ca2+ levels. The data also showed that in the absence of ATP or at high salt, HMM molecules behaved similarly to those in the presence of ATP at high Ca2+. These results suggest that scallop myosin heads can exist in two conformations (heads down towards the tail and heads up away from the tail) and that the equilibrium between these two conformations is altered by the concentrations of salt, ATP and Ca2+. However, the equilibrium between the two forms appears to be too slow to be involved in regulating contraction. The 'heads-down' configuration may instead be related to the inactive, folded (10S) form of scallop myosin and possibly involved in filament assembly during development.

Role of gizzard myosin light chains in calcium binding

The contraction of molluscan and vertebrate smooth muscles is regulated by myosin. Although the myosin and its associated two subunits, the regulatory light chain and the essential light chain, constitute the Ca2+ regulatory system in both types of muscles, the mechanisms by which Ca2+ signal is transduced are quite different. In molluscan muscles, the direct binding of Ca2+ to the regulatory system triggers muscle contraction. In vertebrate smooth muscles, however, phosphorylation of the regulatory light chain is the major triggering mechanism. We measured Ca2+ binding in gizzard myosin and in hybrids of scallop myosin containing gizzard regulatory light chain or in hybrids of scallop regulatory domain containing gizzard essential light chain. Isolated chicken gizzard myosin did not bind Ca2+ in the range of pCa 8.0 to 5.0 in the presence of 2 mM MgCl2, supporting the lack of the specific Ca(2+)-binding site in gizzard myosin. Phosphorylation of the regulatory light chain did not generate a specific (Ca2+)-binding site. The hybrid scallop myosin containing gizzard regulatory light chain showed a similar Ca2+ binding as native scallop myosin with a one to one stoichiometry of Ca2+ to myosin head saturating at about pCa 6.0 at pH 7.6. In contrast, the hybrid scallop regulatory domain containing gizzard essential light chain did not bind Ca2+ either at pCa 6.0 or at pCa 8.0. Control preparations reconstituted with scallop essential light chains bound 0.69 mol per mol Ca2+ at pCa 6.0 with no binding at pCa 8.0.(ABSTRACT TRUNCATED AT 250 WORDS)

Formation and properties of smooth muscle myosin 20-kDa light chain-skeletal muscle myosin hybrids and photocrosslinking from the maleimidylbenzophenone-labeled light chain to the heavy chain

Experimental conditions which permit the exchange of smooth muscle 20-kDa light chain into skeletal muscle myosin are described. The hybridization does not result in the regulation of actin-activated ATPase activity of the hybrid myosin by smooth light chain phosphorylation. Further, the KCl dependence of the Mg-ATPase activity of the hybrid was similar to that of skeletal muscle myosin. The dephosphorylation of the smooth light chain in the hybrid did not induce a conformational change in the hybrid from the 6 S to the 10 S state, thereby indicating that the conformational transition is dependent also on the nature of the heavy chain subunit. Exchange of the smooth light chain premodified at its Cys-108 by photolabile 4-(N-maleimido)benzophenone and photolysis resulted in crosslinking to the heavy chain subunit. Immunopeptide mapping using a monoclonal antibody against residues 1-23 at the N-terminus of the skeletal muscle myosin heavy chain identified the location of the photocrosslinking site to be beyond 92 kDa away from the N-terminus.