L-2-amino-4-phosphonobutyrate (L-AP4) is an agonist at the type iv metabotropic glutamate receptor which is negatively coupled to adenylate cyclase

Glutamate and L-AP4 inhibited forskolin-stimulated cyclic AMP (cAMP) production in baby hamster kidney (BHK) cells transfected with the type IV metabotropic receptor (mGluR4). In situ hybridization revealed a high level of mRNA for the mGluR4 in the entorhinal cortex, but not in the dentate gyrus. These data demonstrate that mGluR4 receptors are negatively coupled to the cAMP cascade, and suggest that the mGluR4 receptor may be the previously described presynaptic L-AP4 receptor.

A pharmacological characterization of the mGluR1 alpha subtype of the metabotropic glutamate receptor expressed in a cloned baby hamster kidney cell line

The pharmacological specificity of the mGluR1 alpha subtype of the metabotropic glutamate receptor (mGluR) was examined in a cloned baby hamster kidney cell line (BHK-ts13) measuring [3H]glutamate binding and inositol phosphate (PI) hydrolysis. PI-hydrolysis was maximally stimulated by quisqualate (1112 +/- 105% of basal), glutamate (1061 +/- 70% of basal), ibotenate (1097 +/- 115% of basal) and beta-N-methylamino-L-alanine (BMAA) (1010 +/- 104% of basal). In contrast, the maximal stimulation of PI-hydrolysis by (1S,3R)-1-amino-cyclopentane-1,3-dicarboxylic acid (t-ACPD) was only 673 +/- 78% of the basal level. The relative order of potency was quisqualate > glutamate > ibotenate > t-ACPD > BMAA. Agonist-stimulated PI-hydrolysis was attenuated (25 +/- 4% inhibition) by L-2-amino-3-phosphonopropionic acid and partially blocked (44 +/- 7%) by pertussis toxin treatment. Saturation binding studies with [3H]glutamate on membranes prepared from BHK-ts13 cells expressing the mGluR1 alpha subtype showed that glutamate binds to a single affinity state of this receptor with a limited capacity (Kd = 296 nM, Bmax = 0.8 pmol/mg protein). In competition experiments, [3H]glutamate was displaced by quisqualate, glutamate, ibotenate, t-ACPD and BMAA with a rank order of potency similar to that found for stimulation of PI-hydrolysis.

Changes in metabotropic glutamate receptor mRNA levels following global ischemia: increase of a putative presynaptic subtype (mGluR4) in highly vulnerable rat brain areas

Metabotropic glutamate receptors mediate their intracellular response by coupling to G proteins and may be divided into three subfamilies: mGluR1 and mGluR5, which stimulate phosphatidylinositol hydrolysis; mGluR2 and mGluR3, which are negatively coupled to cyclic AMP formation; and mGluR4 and mGluR6, which also inhibit forskolin-stimulated cyclic AMP formation. The mGluR4 subtypes may represent L-2-amino-4-phosphonobutyrate-sensitive presynaptic autoreceptors, and two alternatively spliced variants of the mGluR4 coding for two receptors with different C termini have been identified. Using in situ hybridization, we measured the levels of mGluR1-mGluR5 mRNA in regions of the rat brain 24 h after transient global ischemia, a time point when no neuronal damage can yet be observed morphologically. In the hippocampus, the mRNA levels for mGluR1, mGluR2, and mGluR5 were decreased, mGluR3 mRNA levels were unchanged, and the mGluR4 mRNA levels were strongly increased. The strongest increase appeared to be in the mRNA encoding mGluR4b. The mGluR4 mRNA was also increased in the parietal cortex, whereas the ventral posteromedial thalamic nucleus showed a small decrease in its mRNA content. These results suggest that vulnerable neurons react to an increased extracellular glutamate concentration by differential regulation of the mRNA for pre- and postsynaptically located metabotropic glutamate receptors.

Cloning and characterization of a metabotropic glutamate receptor, mGluR4b

An alternative spliced variant of metabotropic glutamate receptor subtype mGluR4a, termed mGluR4b was isolated from a rat cDNA library. Subtype mGluR4b was identical to the previously described mGluR4a, except for the last 64 amino acids in the C-terminal region in which were replaced by 135 new amino acids in mGluR4b. Recombinant baculoviruses coding for mGluR4a and mGluR4b were expressed in Spodoptera frugiperda, Sf-9, insect cells and characterized pharmacologically by measuring [3H]-L-2-amino-4-phosphonobutyrate ([3H]-L-AP4) binding and second messenger formation. [3H]-L-AP4 binding to membranes prepared from Sf-9 cells expressing mGluR4a and mGluR4b revealed respective affinities (Kd) of 480 and 360 nM and maximal binding densities (Bmax) of 4.2 and 0.8 pmol/mg protein. The ligand selectivity of mGluR4a and mGluR4b was similar: L-AP4 > L-serine-O-phosphate > L-glutamate > L-2-amino 2-methyl-4-phosphonobutyrate > (1S,3R)-1-aminocyclopentane-1,3-dicarboxylate > or = quisqualate. A decrease in the affinity of [3H]-L-AP4 was observed in the presence of 0.1 mM guanosine 5'-O-(3-thio)trisphosphate-gamma-S, indicating that mGluR4a and mGluR4b were functionally coupled to G-proteins in Sf-9 cells. Agonists of mGluR4 caused a minor decrease in forskolin-induced cAMP formation in Sf-9 cells expressing either mGluR4a or mGluR4b, suggesting that both receptors are coupled to adenylate cyclase in an inhibitory manner. Thus, mGluR4a and mGluR4b share a common signal transduction pathway and pharmacology when expressed in Sf-9 insect cells.

Characterization of tightly associated smooth muscle myosin-myosin light-chain kinase-calmodulin complexes

A current popular model to explain phosphorylation of smooth muscle myosin (SMM) by myosin light-chain kinase (MLCK) proposes that MLCK is bound tightly to actin but weakly to SMM. We found that MLCK and calmodulin (CaM) co-purify with unphosphorylated SMM from chicken gizzard, suggesting that they are tightly bound. Although the MLCK:SMM molar ratio in SMM preparations was well below stoichiometric (1:73+/-9), the ratio was approximately 23-37% of that in gizzard tissue. Fifteen to 30% of MLCK was associated with CaM at approximately 1 nM free [Ca(2+)]. There were two MLCK pools that bound unphosphorylated SMM with K(d) approximately 10 and 0.2 microM and phosphorylated SMM with K(d) approximately 20 and 0.2 microM. Using an in vitro motility assay to measure actin sliding velocities, we showed that the co-purifying MLCK-CaM was activated by Ca(2+) and phosphorylation of SMM occurred at a pCa(50) of 6.1 and at a Hill coefficient of 0.9. Similar properties were observed from reconstituted MLCK-CaM-SMM. Using motility assays, co-sedimentation assays, and on-coverslip enzyme-linked immunosorbent assays to quantify proteins on the motility assay coverslip, we provide strong evidence that most of the MLCK is bound directly to SMM through the telokin domain and some may also be bound to both SMM and to co-purifying actin through the N-terminal actin-binding domain. These results suggest that this MLCK may play a role in the initiation of contraction.

The kinetics underlying the velocity of smooth muscle myosin filament sliding on actin filaments in vitro

Actin-myosin interactions are well studied using soluble myosin fragments, but little is known about effects of myosin filament structure on mechanochemistry. We stabilized unphosphorylated smooth muscle myosin (SMM) and phosphorylated smooth muscle myosin (pSMM) filaments against ATP-induced depolymerization using a cross-linker and attached fluorescent rhodamine (XL-Rh-SMM). Electron micrographs showed that these side polar filaments are very similar to unmodified filaments. They are ~0.63 μm long and contain ~176 molecules. Rate constants for ATP-induced dissociation and ADP release from acto-myosin for filaments and S1 heads were similar. Actin-activated ATPases of SMM and XL-Rh-SMM were similarly regulated. XL-Rh-pSMM filaments moved processively on F-actin that was bound to a PEG brush surface. ATP dependence of filament velocities was similar to that for solution ATPases at high [actin], suggesting that both processes are limited by the same kinetic step (weak to strong transition) and therefore are attachment- limited. This differs from actin sliding over myosin monomers, which is primarily detachment-limited. Fitting filament data to an attachment-limited model showed that approximately half of the heads are available to move the filament, consistent with a side polar structure. We suggest the low stiffness subfragment 2 (S2) domain remains unhindered during filament motion in our assay. Actin-bound negatively displaced heads will impart minimal drag force because of S2 buckling. Given the ADP release rate, the velocity, and the length of S2, these heads will detach from actin before slack is taken up into a backwardly displaced high stiffness position. This mechanism explains the lack of detachment- limited kinetics at physiological [ATP]. These findings address how nonlinear elasticity in assemblies of motors leads to efficient collective force generation.

Diffusion of myosin light chain kinase on actin: A mechanism to enhance myosin phosphorylation rates in smooth muscle

Smooth muscle myosin (SMM) light chain kinase (MLCK) phosphorylates SMM, thereby activating the ATPase activity required for muscle contraction. The abundance of active MLCK, which is tightly associated with the contractile apparatus, is low relative to that of SMM. SMM phosphorylation is rapid despite the low ratio of MLCK to SMM, raising the question of how one MLCK rapidly phosphorylates many SMM molecules. We used total internal reflection fluorescence microscopy to monitor single molecules of streptavidin-coated quantum dot-labeled MLCK interacting with purified actin, actin bundles, and stress fibers of smooth muscle cells. Surprisingly, MLCK and the N-terminal 75 residues of MLCK (N75) moved on actin bundles and stress fibers of smooth muscle cell cytoskeletons by a random one-dimensional (1-D) diffusion mechanism. Although diffusion of proteins along microtubules and oligonucleotides has been observed previously, this is the first characterization to our knowledge of a protein diffusing in a sustained manner along actin. By measuring the frequency of motion, we found that MLCK motion is permitted only if acto-myosin and MLCK-myosin interactions are weak. From these data, diffusion coefficients, and other kinetic and geometric considerations relating to the contractile apparatus, we suggest that 1-D diffusion of MLCK along actin (a) ensures that diffusion is not rate limiting for phosphorylation, (b) allows MLCK to locate to areas in which myosin is not yet phosphorylated, and (c) allows MLCK to avoid getting "stuck" on myosins that have already been phosphorylated. Diffusion of MLCK along actin filaments may be an important mechanism for enhancing the rate of SMM phosphorylation in smooth muscle.

Modification of interface between regulatory and essential light chains hampers phosphorylation-dependent activation of smooth muscle myosin

We examined the regulatory importance of interactions between regulatory light chain (RLC), essential light chain (ELC), and adjacent heavy chain (HC) in the regulatory domain of smooth muscle heavy meromyosin. After mutating the HC, RLC, and/or ELC to disrupt their predicted interactions (using scallop myosin coordinates), we measured basal ATPase, V(max), and K(ATPase) of actin-activated ATPase, actin-sliding velocities, rigor binding to actin, and kinetics of ATP binding and ADP release. If unphosphorylated, all mutants were similar to wild type showing turned-off behaviors. In contrast, if phosphorylated, mutation of RLC residues smM129Q and smG130C in the F-G helix linker, which interact with the ELC (Ca(2+) binding in scallop), was sufficient to abolish motility and diminish ATPase activity, without altering other parameters. ELC mutations within this interacting ELC loop (smR20M and smK25A) were normal, but smM129Q/G130C-R20M or -K25A showed a partially recovered phenotype suggesting that interaction between the RLC and ELC is important. A molecular dynamics study suggested that breaking the RLC/ELC interface leads to increased flexibility at the interface and ELC-binding site of the HC. We hypothesize that this leads to hampered activation by allowing a pre-existing equilibrium between activated and inhibited structural distributions (Vileno, B., Chamoun, J., Liang, H., Brewer, P., Haldeman, B. D., Facemyer, K. C., Salzameda, B., Song, L., Li, H. C., Cremo, C. R., and Fajer, P. G. (2011) Broad disorder and the allosteric mechanism of myosin II regulation by phosphorylation. Proc. Natl. Acad. Sci. U.S.A. 108, 8218-8223) to be biased strongly toward the inhibited distribution even when the RLC is phosphorylated. We propose that an important structural function of RLC phosphorylation is to promote or assist in the maintenance of an intact RLC/ELC interface. If the RLC/ELC interface is broken, the off-state structures are no longer destabilized by phosphorylation.

Myosin light chain kinase steady-state kinetics: comparison of smooth muscle myosin II and nonmuscle myosin IIB as substrates

Myosin light chain kinase (MLCK) phosphorylates S19 of the myosin regulatory light chain (RLC), which is required to activate myosin's ATPase activity and contraction. Smooth muscles are known to display plasticity in response to factors such as inflammation, developmental stage, or stress, which lead to differential expression of nonmuscle and smooth muscle isoforms. Here, we compare steady-state kinetics parameters for phosphorylation of different MLCK substrates: (1) nonmuscle RLC, (2) smooth muscle RLC, and heavy meromyosin subfragments of (3) nonmuscle myosin IIB, and (4) smooth muscle myosin II. We show that MLCK has a ~2-fold higher k_cat for both smooth muscle myosin II substrates compared with nonmuscle myosin IIB substrates, whereas K_m values were very similar. Myosin light chain kinase has a 1.6-fold and 1.5-fold higher specificity (k_cat /K_m ) for smooth versus nonmuscle-free RLC and heavy meromyosin, respectively, suggesting that differences in specificity are dictated by RLC sequences. Of the 10 non-identical RLC residues, we ruled out 7 as possible underlying causes of different MLCK kinetics. The remaining 3 residues were found to be surface exposed in the N-terminal half of the RLC, consistent with their importance in substrate recognition. These data are consistent with prior deletion/chimera studies and significantly add to understanding of MLCK myosin interactions.

SIGNIFICANCE OF THE STUDY

Phosphorylation of nonmuscle and smooth muscle myosin by myosin light chain kinase (MLCK) is required for activation of myosin's ATPase activity. In smooth muscles, nonmuscle myosin coexists with smooth muscle myosin, but the two myosins have very different chemo-mechanical properties relating to their ability to maintain force. Differences in specificity of MLCK for different myosin isoforms had not been previously investigated. We show that the MLCK prefers smooth muscle myosin by a significant factor. These data suggest that nonmuscle myosin is phosphorylated more slowly than smooth muscle myosin during a contraction cycle.

Kinetics of myosin light chain kinase activation of smooth muscle myosin in an in vitro model system

During activation of smooth muscle contraction, one myosin light chain kinase (MLCK) molecule rapidly phosphorylates many smooth muscle myosin (SMM) molecules, suggesting that muscle activation rates are influenced by the kinetics of MLCK-SMM interactions. To determine the rate-limiting step underlying activation of SMM by MLCK, we measured the kinetics of calcium-calmodulin (Ca²⁺CaM)-MLCK-mediated SMM phosphorylation and the corresponding initiation of SMM-based F-actin motility in an in vitro system with SMM attached to a coverslip surface. Fitting the time course of SMM phosphorylation to a kinetic model gave an initial phosphorylation rate, kp(o), of ~1.17 heads s⁻¹ MLCK⁻¹. Also, we measured the dwell time of single streptavidin-coated quantum dot-labeled MLCK molecules interacting with surface-attached SMM and phosphorylated SMM using total internal reflection fluorescence microscopy. From these data, the dissociation rate constant from phosphorylated SMM was 0.80 s⁻¹, which was similar to the kp(o) mentioned above and with rates measured in solution. This dissociation rate was essentially independent of the phosphorylation state of SMM. From calculations using our measured dissociation rates and Kd values, and estimates of SMM and MLCK concentrations in muscle, we predict that the dissociation of MLCK from phosphorylated SMM is rate-limiting and that the rate of the phosphorylation step is faster than this dissociation rate. Also, association with SMM (11-46 s⁻¹) would be much faster than with pSMM (<0.1-0.2 s⁻¹). This suggests that the probability of MLCK interacting with unphosphorylated versus phosphorylated SMM is 55-460 times greater. This would avoid sequestering MLCK to unproductive interactions with previously phosphorylated SMM, potentially leading to faster rates of phosphorylation in muscle.

Kinetics of myosin light chain kinase activation of smooth muscle myosin in an in vitro model system

During activation of smooth muscle contraction, one myosin light chain kinase (MLCK) molecule rapidly phosphorylates many smooth muscle myosin (SMM) molecules, suggesting that muscle activation rates are influenced by the kinetics of MLCK-SMM interactions. To determine the rate-limiting step underlying activation of SMM by MLCK, we measured the kinetics of calcium-calmodulin (Ca²⁺CaM)-MLCK-mediated SMM phosphorylation and the corresponding initiation of SMM-based F-actin motility in an in vitro system with SMM attached to a coverslip surface. Fitting the time course of SMM phosphorylation to a kinetic model gave an initial phosphorylation rate, kp(o), of ~1.17 heads s⁻¹ MLCK⁻¹. Also, we measured the dwell time of single streptavidin-coated quantum dot-labeled MLCK molecules interacting with surface-attached SMM and phosphorylated SMM using total internal reflection fluorescence microscopy. From these data, the dissociation rate constant from phosphorylated SMM was 0.80 s⁻¹, which was similar to the kp(o) mentioned above and with rates measured in solution. This dissociation rate was essentially independent of the phosphorylation state of SMM. From calculations using our measured dissociation rates and Kd values, and estimates of SMM and MLCK concentrations in muscle, we predict that the dissociation of MLCK from phosphorylated SMM is rate-limiting and that the rate of the phosphorylation step is faster than this dissociation rate. Also, association with SMM (11-46 s⁻¹) would be much faster than with pSMM (<0.1-0.2 s⁻¹). This suggests that the probability of MLCK interacting with unphosphorylated versus phosphorylated SMM is 55-460 times greater. This would avoid sequestering MLCK to unproductive interactions with previously phosphorylated SMM, potentially leading to faster rates of phosphorylation in muscle.

Biochemistry of smooth muscle myosin light chain kinase

The smooth muscle isoform of myosin light chain kinase (MLCK) is a Ca(2+)-calmodulin-activated kinase that is found in many tissues. It is particularly important for regulating smooth muscle contraction by phosphorylation of myosin. This review summarizes selected aspects of recent biochemical work on MLCK that pertains to its function in smooth muscle. In general, the focus of the review is on new findings, unresolved issues, and areas with the potential for high physiological significance that need further study. The review includes a concise summary of the structure, substrates, and enzyme activity, followed by a discussion of the factors that may limit the effective activity of MLCK in the muscle. The interactions of each of the many domains of MLCK with the proteins of the contractile apparatus, and the multi-domain interactions of MLCK that may control its behaviors in the cell are summarized. Finally, new in vitro approaches to studying the mechanism of phosphorylation of myosin are introduced.