Smooth muscle myosin as a calmodulin binding protein. Affinity increase on filament assembly

Purification of Myosin from Bovine Tracheal Smooth Muscle, Filament Formation and Endogenous Association of Its Regulatory Complex

Dynamic regulation of myosin filaments is a crucial factor in the ability of airway smooth muscle (ASM) to adapt to a wide length range. Increased stability or robustness of myosin filaments may play a role in the pathophysiology of asthmatic airways. Biochemical techniques for the purification of myosin and associated regulatory proteins could help elucidate potential alterations in myosin filament properties of asthmatic ASM. An effective myosin purification approach was originally developed for chicken gizzard smooth muscle myosin. More recently, we successfully adapted the procedure to bovine tracheal smooth muscle. This method yields purified myosin with or without the endogenous regulatory complex of myosin light chain kinase and myosin light chain phosphatase. The tight association of the regulatory complex with the assembled myosin filaments can be valuable in functional experiments. The purification protocol discussed here allows for enzymatic comparisons of myosin regulatory proteins. Furthermore, we detail the methodology for quantification and removal of the co-purified regulatory enzymes as a tool for exploring potentially altered phenotypes of the contractile apparatus in diseases such as asthma.

Self-assembly of smooth muscle myosin filaments: adaptation of filament length by telokin and Mg·ATP

The contractile apparatus of smooth muscle is malleable to accommodate stress and strain exerted on the muscle cell and to maintain optimal contractility. Structural lability of smooth muscle myosin filaments is believed to play an important role in the cell's malleability. However, the mechanism and regulation of myosin filament formation is still poorly understood. In the present in vitro study, using a static light scattering method, length distributions were obtained from suspensions of short myosin filaments (SFs) formed by rapid dilution or long ones (LFs) formed by slow dialysis. The distributions indicated the presence of dynamic equilibriums between soluble myosin and the SFs; i.e.: trimers, hexamers and mini filaments, covering the range up to 0.75 µm. The LFs were more stable, exhibiting favorable sizes at about 1.25, 2.4 and 4.5 µm. More distinct distributions were obtained from filaments adsorbed to a glass surface, by evanescent wave scattering and local electric field enhancement. Addition of telokin (TL) to the suspensions of unphosphorylated SFs resulted in widening of the soluble range, while in the case of the LFs this shift was larger, and accompanied by reduced contribution of the soluble myosin species. Such changes were largely absent in the case of phosphorylated myosin. In contrast, the presence of Mg·ATP resulted in elongation of the filaments and clear separation of filaments from soluble myosin species. Thus, TL and Mg·ATP appeared to modify the distribution of myosin filament lengths, i.e., increasing the lengths in preparing for phosphorylation, or reducing it to aid dephosphorylation.

Myosin assembly of smooth muscle: from ribbons and side polarity to a row polar helical model

After decades of debate over the structure of smooth muscle myosin filaments, it is still unclear whether they are helical, as in all other muscle types, or square in shape. In both cases bipolar building units are proposed, but the deduced cross-bridge arrangements are fundamentally different. The opposite polarity of the adjusting longitudinal rows is proposed for the helical structure, while in the case of square filaments, or myosin ribbons, only their two faces are appositively polarized. Analysis of our unpublished archival data on light meromyosin (LMM) paracrystals and myosin rod assemblies as well as the filaments themselves indicated that the rods were assembled with a 6°-7° tilt angle from the rods' longitudinal axis, in contrast to the lack of tilt in LMM, both exhibiting a 14.3 nm myosin periodicity. Optical diffraction analysis of EM images of the rod assemblies and those of intact myosin confirmed their helical architecture characterized by 28 nm residue translations, 172 nm repeats and 516 nm pitch. A detailed helical model of these filaments was elucidated with bipolar tetramer building units made of two polar trimers. The filaments elongate at their two ends in a head-to-head manner, enabling targeted cross-bridge polarity of the adjacent rows, in the form of a unique Boerdijk-Coxeter type helix, similar to that of collagen or desmin fibers, with the covalent links replaced by a head-to-head clasp.

Phosphorylation of caldesmon by myosin light chain kinase increases its binding affinity for phosphorylated myosin filaments

Phosphorylation of myosin by myosin light chain kinase (MLCK) is essential for smooth muscle contraction. In this study we show that caldesmon (CaD) is also phosphorylated in vitro by MLCK. The phosphorylation is calcium- and calmodulin (CaM)-dependent and requires a MLCK concentration close to that found in vivo. On average, approximately 2 mol P i per mol of CaD are incorporated at Thr-626 and Thr-693, with additional partial phosphorylation at Ser-658 and Ser-702. The phosphorylation rate for CaD is 20- to 50-fold slower than that for filamentous myosin; faster relative rates were obtained with CaD added to purified actomyosin or myosin preparations containing endogenous MLCK/CaM complex. Addition of CaM also augmented CaD phosphorylation. We further demonstrate that [32P] labeled CaD binds much more readily to phosphorylated filamentous myosin than to unphosphorylated myosin. For actomyosin, CaD binding affinity doubles after myosin phosphorylation, without a significant change in binding stoichiometry (approx. one CaD per myosin molecule). Unphosphorylated CaD is ineffective in competing with the phosphorylated protein for the binding site(s) on myosin filaments. The ATPase activity of reconstituted actomyosin is inhibited by unphosphorylated CaD, and this inhibition was removed by CaD phosphorylation. Our results suggest that CaD phosphorylation plays a role in modifying actomyosin interaction in vivo, particularly during prolonged muscle activation.

Modulation of myosin filament activation by telokin in smooth muscle liberation of myosin kinase and phosphatase from supramolecular complexes

The mechanism of telokin action on reversible phosphorylation of turkey gizzard myosin was investigated using a native-like filamentous myosin. This myosin contained endogenous calmodulin (CaM) and myosin light chain kinase (MLCK) at a molar ratio to myosin of about 1 to 40 or less depending on the initial extractions conditions. These levels were sufficient to fully phosphorylate myosin within 20-40 s or less after addition of [gamma-32P]ATP, but when the ATP was depleted, they became dephosphorylated indicating the presence of myosin light chain phosphatase (MLCP). Addition of telokin at the 1 to 1 or higher molar ratio to myosin caused a three- to five-fold inhibition of the initial phosphorylation rates (without reduction of the overall extent of phosphorylation) and produced a similar increase in the rate of dephosphorylation. The inhibition was also observed for myosin filaments free of MLCK and CaM together with constitutively active MLCKs produced by digestion, or by expression of a truncated mammalian kinase as well as for the wild-type enzyme. Thus, neither N- nor C-terminal of MLCK was necessary for interaction of myosin with telokin and the inhibition resulted from telokin-induced change of myosin head configuration within the filament that prevented their ordered, paracrystaline-like, aggregation. Sedimentation of the filamentous myosin in glycerol gradients showed that this change made the filaments less compact and facilitated release of the endogenous MLCK/CaM complex. For a mixture of the filaments with or without the complex, the configuration change resulted in an increase of the phosphorylation rate but not in its inhibition. The increase of the rate resulting from the liberation of the complex was also observed in mixtures of the filamentous myosin with added isolated regulatory light chain (ReLC) or soluble myosin head subfragment. This observation reinforces the above conclusions. The acceleration of the MLCP activity by telokin was shown to result from dissociation of its catalytic subunit from a MLCK/MLCP complex bound to the filamentous myosin. Analogous desensitizing effects of telokin were also demonstrated for the contraction and relaxation cycle of Triton-skinned fibers from guinea pig Teania coli. Taken together, our results indicate that telokin acted as an effective modulator or chaperone of the myosin filament and a scheme for its action in smooth muscle was proposed.

Vectorial activation of smooth muscle myosin filaments and its modulation by telokin

Smooth muscle myosin copurifies with myosin light chain kinase (MLCK) and calmodulin (CaM) as well as with variable amounts of myosin phosphatase. Therefore, myosin filaments formed in vitro also contain relatively high levels of these enzymes. Thus these filaments may be considered to be native-like because they are similar to those expected to exist in vivo. These endogenous enzymes are present at high concentrations relative to myosin, sufficient for rapid phosphorylation and dephosphorylation of the filaments at rates comparable to those observed for contraction and relaxation in intact muscle strips. The phosphorylation by MLCK/CaM complex appears to exhibit some directionality and is not governed by a random diffusional process. For the mixtures of myosin filaments with and without the endogenous MLCK/CaM complex, the complex preferentially phosphorylates its own parent filament at a higher rate than the neighboring filaments. This selective or vectorial-like activation is lost or absent when myosin filaments are dissolved at high ionic strength. Similar vectorial-like activation is exhibited by the reconstituted filament suspensions, but the soluble systems composed of isolated regulatory light chain or soluble myosin head subfragments exhibit normal diffusional kinetic behavior. At physiological concentrations, kinase related protein (telokin) effectively modulates the activation process by reducing the phosphorylation rate of the filaments without affecting the overall phosphorylation level. This results from telokin-induced liberation of the active MLCK/CaM complex from the filaments, so that the latter can also activate the neighboring filaments via a slower diffusional process. When this complex is bound at insufficient levels, this actually results in acceleration of the initial phosphorylation rates. In short, I suggest that in smooth muscle, telokin plays a chaperone role for myosin and its filaments.

Vectorial phosphorylation of filamentous smooth muscle myosin by calmodulin and myosin light chain kinase complex

Vertebrate smooth muscle myosin extracted from myofibrils and isolated via filament assembly was co-purified with calmodulin (CaM) and myosin light chain kinase (MLCK) which are tightly associated with the filament architecture and, therefore, it may be considered as a native-like preparation. These endogenous contaminates also co-precipitated with a native-like actomyosin, for both cases, at levels sufficient to fully phosphorylate myosin within 10-20 s after addition of ATP and calcium, although their molar ratio to myosin was only about 1 to 100. Phosphorylation progress curves obtained from mixtures of the native-like, and CaM- and MLCK-free filaments indicated that the CaM/MLCK complex preferentially phosphorylated its parent filaments and, as result, the whole myosin present was not maximally phosphorylated. Solubilization of the filaments' mixtures at high ionic strength resulted in slower phosphorylation rates but with maximal phosphorylation levels being attainable. Similar observations were made on the filamentous myosin system reconstituted from the kinase- and CaM-free myosin with added purified MLCK and CaM as well as on the native-like myosin from which only one of these endogenous contaminates was removed by affinity chromatography. These data indicated that not only the MLCK but also CaM was necessary for the observed preferential phosphorylation kinetics. Thus, the native-like filamentous myosin appeared to be phosphorylated by some kind of vectorial mechanism. Similar experiments were carried out on the native-like actomyosin where these vectorial effects were even more pronounced.

Mechanism of phosphorylation of the regulatory light chain of myosin from tarantula striated muscle

Contraction is modulated in many striated muscles by Ca2+-calmodulin dependent phosphorylation of the myosin regulatory light chain (RLC) by myosin light chain kinase. We have investigated the biochemical mechanism of RLC phosphorylation in tarantula muscle to better understand the basis of myosin-linked regulation. In an earlier study it was concluded that the RLC occurred as two species, both of which could be phosphorylated, potentiating contraction. Here we present evidence that only a single species exists, and that this can be phosphorylated at one or two sites. In relaxed muscle we find evidence for a substantial level of basal phosphorylation at the first site. This is augmented on activation, followed by partial phosphorylation of the second site. We find in addition that Ca2+ has a dual effect on light chain phosphorylation, depending on its concentration. At low concentration (relaxing conditions) only basal phosphorylation is observed, while at higher concentrations (activating conditions) RLC phosphorylation is stimulated. At still higher Ca2+ concentrations we find partial inhibition of RLC phosphorylation, suggesting an additional mechanism by which the muscle cell can fine tune contractile activity by controlling the level of free Ca2+.

Enzyme kinetic characterization of the smooth muscle myosin phosphorylating system: activation by calcium and calmodulin and possible inhibitory mechanisms of antagonists

A native-like smooth muscle filamentous myosin system was characterized from an enzyme kinetic point of view. The system contains endogenous myosin light chain kinase (MLCKase) and calmodulin (CM) (A. Sobieszek, J. Muscle Res. Cell Motil. 11 (1990) 114-124) and is, therefore, well suited for testing the action of CM-antagonists or other inhibitory compounds. However, this has not been done due to its complexity. The characterization of the system includes: (1) derivation of a relationship for rate of myosin phosphorylation in terms of total CM, free Ca2+ and total MLCKase concentrations, which includes only three binding constants; and (2) derivation of relationships between fractional inhibition rate (vi/vo) and total inhibitor concentration (It) which cover most of the inhibitory mechanisms applicable to the myosin system or to other CM-dependent enzymes. The three binding constants were subsequently evaluated from experimental data for filamentous myosin and for its isolated regulatory light chain (ReLC) using a non-linear regression software. They indicated differences in the interaction of myosin filament with the active CM-MLCKase complex in comparison to that of the isolated ReLC. The derived vi/vo versus It relationships, together with the software, make it possible to evaluate the inhibition constants and binding stoichiometries of CM-antagonists and other compounds inhibiting myosin phosphorylation. This approach was successfully applied to experimental data on inhibition of MLCKase by amiloride, cadmium, or CM-binding peptide (M-12) for simple mechanisms. For more complex mechanisms, inhibition by calmidozolium, trifluoperazine or melittin, the analysis showed that only calmidozolium acted specifically at the CM level in a multiple-site activator-depletion mechanism. Melittin and trifluoperazine inhibited the phosphorylation rate by a novel substrate-and-activator depletion mechanism, in which additional inhibition of the substrate resulted in the removal of the inhibition at lower range of the antagonists' concentration.

Kinase-related protein (telokin) is phosphorylated by smooth-muscle myosin light-chain kinase and modulates the kinase activity

Telokin is an abundant smooth-muscle protein with an amino acid sequence identical with that of the C-terminal region of smooth-muscle myosin light-chain kinase (MLCK), although it is expressed as a separate protein [Gallagher and Herring (1991) J. Biol. Chem. 266, 23945-23952]. Here we demonstrate that telokin is also similar to smooth-muscle myosin regulatory light chain (ReLC) not only in its gross physical properties but also as an MLCK substrate. Telokin was slowly phosphorylated by MLCK in the presence of Ca2+ and calmodulin and could be readily dephosphorylated by myosin light-chain phosphatase. A threonine residue was phosphorylated with up to 0.25 mol/mol stoichiometry. This low stoichiometry, together with the observed dimerization of telokin [Sobieszek and Nieznanski (1997) Biochem. J. 322, 65-71], indicates that the telokin dimer was acting as the substrate with a single protomer being phosphorylated. Our enzyme kinetic analysis of the phosphorylation reaction confirms this interpretation. Because telokin phosphorylation also required micromolar concentrations of MLCK, which also facilitates the formation of kinase oligomers, we concluded that the oligomers are interacting with telokin. Thus it seems that telokin modulates the phosphorylation rate of myosin filaments by a mechanism that includes the direct or indirect inhibition of the kinase active site by the telokin dimer, and that removal of the inhibition is controlled by slow phosphorylation of the telokin dimer, which results in MLCK dimerization.

Oligomerization of smooth muscle myosin light chain kinase and its modifications by melittin and calmodulin

The catalytic activity of smooth muscle myosin light chain kinase (MLCKase) requires the presence of calcium and calmodulin [CM; J. T. Stull et al. (1993) Molecular and Cellular Biochemistry, Vols. 127/128, pp. 229-237] and can also be modified through its own oligomerization [E. B. Babiychuk et al. (1995) Biochemistry, Vol. 34, pp. 6366-6372]. In the present report we demonstrate that melittin, one of the most potent CM antagonists, interacted reversibly with the MLCKase apoenzyme with affinities comparable to those of CM and influenced the oligomeric state of the kinase. At low melittin to kinase ratios the kinase formed insoluble oligomers (aggregates) while at higher melittin concentrations it existed predominantly as soluble oligomers revealed by cross-linking as octamers and hexamers. The kinase alone exhibited similar biphasic solubility within a 5-30 microM range and its solubility was strongly influenced by the ionic strength of the medium. Melittin was also found to promote both the aggregation of the purified 24-kDa C-terminal fragment of the kinase and its analogue telokin, as well as of myosin light chains, but had no effect on the solubility of bovine serum albumin, caldesmon, or calmodulin. These data and our cross-linkage experiments indicate that the insoluble kinase oligomers arose via melittin-induced aggregation of the kinase dimers in which the relokin-like domain played a main role. The soluble oligomers, in turn, were formed after saturation of the kinase with melittin, which resulted in a weakening of the interaction between the protomers with an increase of the long-range order within the oligomers. This interpretation is consistent with the observed effects of melittin on MLCKase catalytic and autocatalytic activities. At concentrations of melittin required to produce soluble oligomers, the binding of the kinase to myosin filaments was considerably enhanced. A plausible mechanism for the formation of the soluble oligomers and aggregates is suggested and its relation to the possible MLCKase assemblies discussed in terms of a model.

Smooth muscle myosin light chain kinase, supramolecular organization, modulation of activity, and related conformational changes

It has recently been suggested that activation of smooth muscle myosin light chain kinase (MLCK) can be modulated by formation of supramolecular structures (Sobieszek, A. 1991. Regulation of smooth muscle myosin light chain kinase. Allosteric effects and co-operative activation by CaM. J. Mol. Biol. 220:947-957). The present light scattering data demonstrate that the inactive (calmodulin-free) MLCK apoenzyme exists in solution as a mixture of oligomeric (2% by weight), dimeric (53%), and monomeric (45%) species at physiological ionic strength (160 mM salt). These long-living assemblies, the lifetime of which was measured by minutes, were in equilibrium with each other. The most likely form of the oligomer was a spiral-like hexamer, the dimensions of which fit very well the helical structure of self-assembled myosin filaments (Sobieszek, A. 1972. Cross-bridges on self-assembled smooth muscle myosin filaments. J. Mol. Biol. 70:741-744). After activation of the kinase by calmodulin (CaM) we could not detect any appreciable changes in the distribution of the kinase species either when the kinase was saturated with CaM or when its molar concentration exceeded that of CaM. Our fluorescent measurements suggest that the earlier observed inhibition of kinase at substoichiometric amounts of CaM (Sobieszek, A., A. Strobl, B. Ortner, and E. Babiychuk. 1993. Ca2+-calmodulin-dependent modification of smooth-muscle myosin light chain kinase leading to its co-operative activation by calmodulin. Biochem. J. 295:405-411) is associated with slow conformational change(s) of the activated (CaM-bound) kinase molecules. Such conformational rearrangements also took place with equimolar kinase to CaM; however, in this case there was no decrease in MLCK activity. The nature of these conformational changes, which are accompanied by reduction of the kinase for CaM affinity, is discussed.

Purification and characterization of a kinase-associated, myofibrillar smooth muscle myosin light chain phosphatase possessing a calmodulin-targeting subunit

A myofibrillar form of smooth muscle myosin light chain phosphatase (MLCPase) was purified from turkey gizzard myofibrils, and it was found to be closely associated with the myosin light chain kinase (MLCKase). For this reason we have named this phosphatase the kinase- and myosin-associated protein phosphatase (KAMPPase). Subunits of the KAMPPase could be identified during the first ion exchange chromatography step. After further purification on calmodulin (CaM) and on thiophosphorylated regulatory myosin light chain affinity columns we obtained either a homogenous preparation of a 37-kDa catalytic (PC) subunit or a mixture of the PC subunit and variable amounts of a 67-kDa targeting (PT) subunit. The PT subunit bound the PC subunit to CaM affinity columns in a Ca2+-independent manner; thus, elution of the subunits required only high salt concentration. Specificity of interaction between these subunits was shown by the following observations: 1) activity of isolated PC subunit, but not of the PTC holoenzyme, was stimulated 10-20-fold after preincubation with 5-50 microM of CoCl2; 2) the pH activity profile of the PC subunit was modified by the PT subunit (the specific activity of the PTC holoenzyme was higher at neutral pH and lower at alkaline pH); and 3) affinity of the holoenzyme for unphosphorylated myosin was 3-fold higher, and for phosphorylated myosin it was 2-fold lower, in comparison with that of the purified PC subunit. KAMPPase was inhibited by okadaic acid (Ki = 250 nM), microcystin-LR (50 nM) and calyculin A (1.5 microM) but not by arachidonic acid or the heat-stable inhibitor (I-2), which suggested that this is a type PP1 or PP2A protein phosphatase.

Purification and characterization of a smooth muscle myosin light chain kinase-phosphatase complex

We show that a myofibrillar form of smooth muscle myosin light chain phosphatase (MLCPase) forms a multienzyme complex with myosin light chain kinase (MLCKase). The stability of the complex was indicated by the copurification of MLCKase and MLCPase activities through multiple steps that included myofibril preparation, gel filtration chromatography, cation (SP-Sepharose BB) and anion (Q-Sepharose FF) exchange chromatography, and affinity purification on calmodulin and on thiophosphorylated regulatory light chain columns. In addition, the purified complex eluted as a single peak from a final gel filtration column in the presence of calmodulin (CaM). Because a similar MLCPase is present in varying amounts in standard preparations of both MLCKase and myosin filaments, we have named it a kinase- and myosin-associated protein phosphatase (KAMPPase). The KAMPPase multienzyme complex was composed of a 37-kDa catalytic (PC) subunit, a 67-kDa targeting (PT) subunit, and MLCKase with or without CaM. The approximate molar ratio of the PC and PT subunits was 1:2 with a variable and usually higher molar content of MLCKase. The targeting role of the PT subunit was directly demonstrated in binding experiments in which the PT subunit bound to both the kinase and to CaM. Its binding to CaM was, however, Ca2+-independent. MLCKase and the PT subunit potentiated activity of the PC subunit when intact myosin was used as the substrate. These data indicated that there is a Ca2+-independent interaction among the MLCPase, MLCKase, and CaM that are involved in the regulation of phosphatase activity.

Telokin (kinase-related protein) modulates the oligomeric state of smooth-muscle myosin light-chain kinase and its interaction with myosin filaments

Telokin, an abundant gizzard protein, inhibited phosphorylation of regulatory light chain when filamentous myosin was used as the substrate but no inhibition was observed with myosin subfragment 1. At physiological telokin-to-myosin molar ratio (1:1), the inhibition amounted to a 3.5-fold reduction in the initial phosphorylation rate whereas at high molar excess of telokin over myosin, we observed an up to 20-fold decrease in this rate. In agreement with previous observations [Shirinsky, Vorotnikow, Birukov, Nanaev, Collinge, Lukas, Sellers and Watterson (1993) J. Biol. Chem. 268, 16578-16583], telokin did not inhibit phosphorylation of the isolated regulatory light chain of myosin and only moderately (35%) inhibited that of heavy meromyosin. To gain a better understanding of the mechanism of this inhibition, we investigated the effects of telokin on the recently described [Babiychuk, Babiychuk and Sobieszek (1995) Biochemistry 34, 6366-6372] oligomeric properties of smooth-muscle myosin light-chain kinase (MLCK). We showed, on the one hand, that telokin rapidly solubilized the large kinase oligomers formed at low ionic strength. With soluble kinase, on the other hand, telokin acted to increase the relative concentration of MLCK dimers and to decrease that of the hexamers and octamers. This, in turn, resulted in a reduction in the amount of MLCK bound to myosin because filamentous myosin appeared to exhibit a higher affinity for the hexamers than for the dimers. Telokin by itself was also shown to dimerize and oligomerize in solution and this oligomerization was greatly enhanced in the presence of MLCK. We suggest that telokin affects myosin phosphorylation by modulation of the oligomeric state of MLCK and its interaction with myosin filaments.

Ca(2+)-calmodulin-dependent modification of smooth-muscle myosin light-chain kinase leading to its co-operative activation by calmodulin

It has recently been shown that at relatively high molar ratios of myosin light-chain kinase (MLCKase) to calmodulin (CM) almost complete inhibition of the kinase activity occurs [Sobieszek (1991) J. Mol. Biol. 220, 947-957]. This inhibition resulted in a highly co-operative activation of MLCKase by CM, whereas the opposite activation (of CM by kinase) was hyperbolic, as expected (unco-operative). This difference in activation was observed only for kinase preparations preincubated with sub-stoichiometric amounts of CM, and only when micromolar concentrations of Ca2+ were present. The inhibitory effect was variable and depended not only on the concentration ratio of kinase to CM but also on the MLCKase preparation. For most of the preparations full inhibition required 5-15 min of preincubation at 25 degrees C and a 3-6-fold molar excess of kinase over CM. The inhibition was reversible, since full activity could be obtained after saturation of the kinase by additional CM. The inhibitory effect did not require ATP (excluding phosphorylation-type modifications of the kinase), and dephosphorylation of the kinase was not involved, since inhibition of an endogenous MLCK phosphatase by microcystin-LR did not decrease the inhibitory effect. Since the co-operative activation by CM was observed for cross-linked MLCKase preparations enriched in kinase dimers, but was absent for the analogous preparations enriched in the oligomers, we concluded that Ca(2+)-CM-dependent changes in the oligomeric state of the kinase were responsible for the modification observed. The exact nature of these modifications remains to be established.

Regulation of smooth muscle myosin light chain kinase. Allosteric effects and co-operative activation by calmodulin

The activation of smooth muscle myosin light chain kinase (MLCKase) by calcium and calmodulin (CM) was investigated over a wide range of concentrations of the enzyme using myosin (MY) or its isolated phosphorylatable light chain (L20) as substrates. The enzyme showed allosteric behavior. The specific phosphorylation activity was dependent on the concentration of MLCKase as well as on the concentrations of both substrates. However, at the lower (nanomolar) range of kinase the corresponding substrate rate relationships were hyperbolic. A high positive level of co-operativity of kinase was also observed for activation by CM in the presence of Ca2+. There was a pronounced CM/Ca-dependent inhibition of MLCKase activity when its molar ratio to CM was four to one or more. These kinetic data suggested that MLCKase could exist in several oligomeric forms, with an inactive high molecular size form and an active low molecular size form (protomers and/or dimers). This conclusion was confirmed by gel filtration studies. CM was not directly involved in the oligomerization process but instead, the oligomeric kinase shared an increased affinity for CM.

Regulation of smooth-muscle myosin-light-chain kinase. Steady-state kinetic studies of the reaction mechanism

The kinetic mechanism of turkey gizzard smooth muscle myosin-light-chain kinase was investigated using the isolated 20-kDa light chain of myosin as substrate. The kinetic and product inhibition patterns of the forward reaction indicated an ordered sequential mechanism in which MgATP bound first, ADP was released last. The order of substrate binding and product release was confirmed independently by competitive, dead-end inhibition patterns obtained using the non-hydrolizable ATP analog adenosine 5'-[beta,gamma-imido]triphosphate. The mechanism was also characterized by a relatively strong product inhibition by ADP and a weak one by phosphorylated 20-kDa light-chain myosin, in addition to a significant inhibition by the latter product via a formation of a dead-end complex. [gamma-32P]ATP in equilibrium with [32P]phosphorylated light chain isotope-exchange data were consistent with the deduced mechanism and with the presence of the latter dead-end complex.