Specific interactions of the alkali light chain 1 in skeletal myosin heads probed by chemical cross-linking

Effect of the N-terminal extension in myosin essential light chain A1 on the mechanism of actomyosin ATP hydrolysis

Myosin essential light chains A1 and A2 are identical isoforms except for an extension of ∼40 amino acids at the N terminus of A1 that binds F-actin. The extension has no bearing on the burst hydrolysis rate (M-ATP → M-ADP-Pi) as determined by chemical quench flow (100 μM isoenzyme). Whereas actomyosin-S1A2 steady state MgATPase (low ionic strength, 20 °C) is hyperbolically dependent on concentration: Vmax 7.6 s-1, Kapp 6.4 μM (F-actin) and Vmax 10.1 s-1, Kapp 5.5 μM (native thin filaments, pCa 4), the relationship for myosin-S1A1 is bimodal; an initial rise at low concentration followed by a decline to one-third the Vmax of S1A2, indicative of more than one rate-limiting step and A1-enforced flux through the slower actomyosin-limited hydrolysis pathway. In double-mixing stopped-flow with an indicator, Ca(II)-mediated activation of Pi dissociation (regulatedAM-ADP-Pi → regulatedAM-ADP + Pi) is attenuated by A1 attachment to thin filaments (pCa 4). The maximum accelerated rates of Pi dissociation are: 81 s-1 (S1A1, Kapp 8.9 μM) versus 129 s-1 (S1A2, Kapp 58 μM). To investigate apomyosin-S1-mediated activation, thin filaments (EGTA) are premixed with a given isomyosin-S1 and double-mixing is repeated with myosin-S1A1 in the first mix. Similar maximum rates of Pi dissociation are observed, 44.5 s-1 (S1A1) and 47.1 s-1 (S1A2), which are lower than for Ca(II) activation. Overall, these results biochemically demonstrate how the longer light chain A1 can contribute to slower contraction and higher force and the shorter version A2 to faster contraction and lower force, consistent with their distribution in different types of striated muscle.

Cardiac ventricular myosin and slow skeletal myosin exhibit dissimilar chemomechanical properties despite bearing the same myosin heavy chain isoform

The myosin II motors are ATP-powered force-generating machines driving cardiac and muscle contraction. Myosin II heavy chain isoform-beta (β-MyHC) is primarily expressed in the ventricular myocardium and in slow-twitch muscle fibers, such as M. soleus. M. soleus-derived myosin II (SolM-II) is often used as an alternative to the ventricular β-cardiac myosin (βM-II); however, the direct assessment of biochemical and mechanical features of the native myosins is limited. By employing optical trapping, we examined the mechanochemical properties of native myosins isolated from the rabbit heart ventricle and soleus muscles at the single-molecule level. We found purified motors from the two tissue sources, despite expressing the same MyHC isoform, displayed distinct motile and ATPase kinetic properties. We demonstrate βM-II was approximately threefold faster in the actin filament-gliding assay than SolM-II. The maximum actomyosin (AM) detachment rate derived in single-molecule assays was also approximately threefold higher in βM-II, while the power stroke size and stiffness of the "AM rigor" crossbridge for both myosins were comparable. Our analysis revealed a higher AM detachment rate for βM-II, corresponding to the enhanced ADP release rates from the crossbridge, likely responsible for the observed differences in the motility driven by these myosins. Finally, we observed a distinct myosin light chain 1 isoform (MLC1sa) that associates with SolM-II, which might contribute to the observed kinetics differences between βM-II and SolM-II. These results have important implications for the choice of tissue sources and justify prerequisites for the correct myosin heavy and light chains to study cardiomyopathies.

Intermolecular Interactions of Myosin Subfragment 1 Induced by the N-Terminal Extension of Essential Light Chain 1

We applied dynamic light scattering (DLS) to compare aggregation properties of two isoforms of myosin subfragment 1 (S1) containing different "essential" (or "alkali") light chains, A1 or A2, which differ by the presence of an N-terminal extension in A1. Upon mild heating (up to 40°C), which was not accompanied by thermal denaturation of the protein, we observed a significant growth in the hydrodynamic radius of the particles for S1(A1), from ~18 to ~600-700 nm, whereas the radius of S1(A2) remained unchanged and equal to ~18 nm. Similar difference between S1(A1) and S1(A2) was observed in the presence of ADP. In contrast, no differences were observed by DLS between these two S1 isoforms in their complexes S1-ADP-BeF_x and S1-ADP-AlF₄^- which mimic the S1 ATPase intermediate states S1*-ATP and S1**-ADP-P_i. We propose that during the ATPase cycle the A1 N-terminal extension can interact with the motor domain of the same S1 molecule, and this can explain why S1(A1) and S1(A2) in S1-ADP-BeF_x and S1-ADP-AlF₄^- complexes do not differ in their aggregation properties. In the absence of nucleotides (or in the presence of ADP), the A1 N-terminal extension can interact with actin, thus forming an additional actin-binding site on the myosin head. However, in the absence of actin, this extension seems to be unable to undergo intramolecular interaction, but it probably can interact with the motor domain of another S1 molecule. These intermolecular interactions of the A1 N-terminus can explain unusual aggregation properties of S1(A1).

Minigenes encoding N‐terminal domains of human cardiac myosin light chain‐1 improve heart function of transgenic rats

In this study we investigated whether the expression of N-terminal myosin light chain-1 (MLC-1) peptides could improve the intrinsic contractility of the whole heart. We generated transgenic rats (TGR) that overexpressed minigenes encoding the N-terminal 15 amino acids of human atrial MLC-1 (TGR/hALC-1/1-15, lines 7475 and 3966) or human ventricular MLC-1 (TGR/hVLC-1/1-15, lines 6113 and 6114) isoforms in cardiomyocytes. Synthetic N-terminal peptides revealed specific actin binding, with a significantly (P<0.01) lower dissociation constant (K(D)) for the hVLC-1/1-15-actin complex compared with the K(D) value of the hALC-1/1-15-actin complex. Using synthetic hVLC-1/1-15 as a TAT fusion peptide labeled with the fluorochrome TAMRA, we observed specific accumulation of the N-terminal MLC-1 peptide at the sarcomere predominantly within the actin-containing I-band, but also within the actin-myosin overlap zone (A-band) in intact adult cardiomyocytes. For the first time we show that the expression of N-terminal human MLC-1 peptides in TGR (range: 3-6 muM) correlated positively with significant (P<0.001) improvements of the intrinsic contractile state of the isolated perfused heart (Langendorff mode): systolic force generation, as well as the rates of both force generation and relaxation, rose in TGR lines that expressed the transgenic human MLC-1 peptide, but not in a TGR line with undetectable transgene expression levels. The positive inotropic effect of MLC-1 peptides occurred in the absence of a hypertrophic response. Thus, expression of N-terminal domains of MLC-1 represent a valuable tool for the treatment of the failing heart.

Fine tuning the myosin motor: the role of the essential light chain in striated muscle myosin

It has long been known that the essential light chain isoform of striated muscle affects the function of the myosin motor. There are two isoforms: A1-type and A2-type that differ by the presence of an extra 40 amino acids at the N-terminus of A1-type light chains. Evidence has accumulated from a variety of experimental techniques that this extension of A1-type light chains makes a direct contact with actin, increasing the overall affinity between myosin and actin and that this interaction is responsible for the modulation of myosin motor function. Some recent work, however, has provided some contradictory data. Experiments using more physiologically relevant forms of myosin have suggested that the effect of the N-terminal region of A1-type light chains may, in some circumstances, be to weaken, rather than strengthen the actin-myosin interaction. Work with transgenic mice in which this region was mutated showed no measurable phenotypic effects on either muscle or whole organism function questioning the in vivo significance of the light chain-actin interaction. It is also possible that the essential light chain has other functions in the cell. There is evidence that the protein may interact with IQGAP, a regulator of the actin cytoskeleton. The consequences of this interaction are unknown. This review aims to summarise the biochemical data on striated muscle myosin essential light chain isoform function and to reconcile it with these recent discoveries.

Interaction of the N-terminal part of the A1 essential light chain with the myosin heavy chain

The kinetics of actin-dependent MgATPase activity of skeletal muscle myosin subfragment 1 (S1) isoform containing the A1 essential light chain differ from those of the S1 isoform containing the A2 essential light chain. The differences are due to the presence of the extra N-terminal peptide comprising 42 amino acid residues in the A1 light chain. This peptide can interact with actin; heretofore, there have no been reports of the direct interaction between this peptide and the heavy chain of S1. Here, using the zero-length cross-linker 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) and S. aureus V8 protease, we show for the first time that the N-terminal part of the A1-light chain can interact with the 22-kDa fragment of the S1 heavy chain. No such interaction has been observed for the S1(A2) isoenzyme. Localization of residues which can possibly react with the cross-linker suggests that the interaction might involve the N-terminal residues of the A1 light chain and the converter region of the heavy chain.

An essential myosin light chain peptide induces supramaximal stimulation of cardiac myofibrillar ATPase activity

The N-terminal region of skeletal myosin light chain-1 (MLC-1) binds to the C terminus of actin, yet the functional significance of this interaction is unclear. We studied a fragment (MLC-pep; residues 5-14) of the ventricular MLC-1. When added to rat cardiac myofibrils, 10 nM MLC-pep induced a supramaximal increase in the MgATPase activity at submaximal Ca2+ levels with no effect at low and maximal Ca2+ levels. A nonsense, scrambled sequence peptide had no effect at any pCa value. MLC-pep did not affect myosin KEDTA and CaATPase activities or actin-activated MgATPase activities in the absence or presence of tropomyosin. The MLC-pep did not alter the ability of troponin I to inhibit MgATPase activity. Moreover, when troponin I and troponin C were extracted from the myofibrils, the MLC-pep lost its ability to stimulate the ATPase rate. This effect was fully restored upon reconstitution of the extracted myofibrils with troponin I-troponin C complex. Thus, activation of MgATPase activity by the peptide required a full complement of thin filament regulatory proteins. Interestingly, the stimulatory effect occurred at a ratio of 4 peptides to 1 thin filament, suggesting that the peptide engages in a highly cooperative process that may involve activation of the entire thin filament.

Localization of a myosin subfragment-1 interaction site on the C-terminal part of actin

The actin-myosin head complex in the rigor state reveals several high-affinity sites on the actin molecule in sequences 18-28 and 40-113. In the presence of Mg(2+)-ATP, participation of the actin N-terminal 1-7 sequence is known to occur. The proximity of the C-terminal region of actin to the A1 light chain of the myosin head [S-1(A1)] (where S-1 is myosin subfragment-1) was described previously. We observed that C-terminal antigenic structures located near Met-305, Met-325 and Met-355 and the C-terminal end (Cys-374) of actin are markedly modified in the presence of S-1(A1), S-1(A2) and scallop S-1 and in the absence of Mg(2+)-ATP. This seems to rule out any important specific involvement of the A1 light chain in the described conformational changes. An S-1-binding site was located in this actin C-terminal region by testing the tryptic CB9 peptide (360-372 sequence) previously implicated in the A1 light chain interaction. This peptide was able to bind well to S-1(A1), S-1(A2) and scallop S-1, but not in the presence of Mg(2+)-pyrophosphate. These results strengthen the hypothesis of a multisite interface between S-1 and actin located in the actin subdomain I.

Characterization of an actin-myosin head interface in the 40-113 region of actin using specific antibodies as probes

Evidence for the participation of the 1-7 and 18-28 N-terminal sequences of actin at different steps of actin-myosin interaction process is well documented in the literature. Cross-linking of the rigor complex between filamentous actin and skeletal-muscle myosin subfragment 1 was accomplished by the carboxy-group-directed zero-length protein cross-linker, 1-ethyl-3-[3-(dimethylamino)propyl]carbodi-imide. After chaotropic depolymerization and thrombin digestion, which cleaves only actin, the covalent complex with Mr 100,000 was characterized by PAGE. The linkage was identified as being between myosin subfragment 1 (S-1) heavy chain and actin-(1-28)-peptide. The purified complex retained in toto its ability to combine reversibly with fresh filamentous actin, but showed a decrease in the Vmax. of actin-dependent Mg2(+)-ATPase. By using e.l.i.s.a., S-1 was observed to bind to coated monomeric actin or its 1-226 N-terminal peptide. This interaction strongly interfered with the binding of antibodies directed against the 95-113 actin sequence. Moreover, S-1 was able to bind with coated purified actin-(40-113)-peptide. Finally, antibodies directed against the 18-28 and 95-113 actin sequence, which strongly interfered with S1 binding, were unable to compete with each other. These results suggest that two topologically independent regions are involved in the actin-myosin interface: one located in the conserved 18-28 sequence and the other near residues 95-113, including the variable residue at position 89. Other experiments support the 'multisite interface model', where the two actin sites could modulate each other during S-1 interaction.

Influence of nucleotide on chemical crosslinking between alkali light chains and the heavy chain of myosin subfragment 1

When chymotryptic myosin subfragment 1 (S1) of fast skeletal muscle myosin is treated with dithiobis(succinimidylpropionate) (DSP), the alkali light chains A1 and A2 become intramolecularly crosslinked to the N-terminal 27 kDa fragment of the S1 heavy chain (Labbé et al. (1981) Biochem. Biophys. Res. Commun. 102, 466-475). The results presented here show that in the presence of MgATP the efficiency of the crosslinking is markedly reduced. The results may indicate a nucleotide-induced structural rearrangement within the myosin head. It was also observed that crosslinking depressed the nucleotide-promoted tryptic conversion of the 27 kDa fragment to its 22 kDa derivative, suggesting that the crosslinks are in the vicinity of the additional tryptic cleavage site in the 27 kDa fragment or that the crosslinking prevents nucleotide-induced conformational changes in this region of the S1 heavy chain.

Characterization of a rat myosin alkali light chain gene expressed in ventricular and slow twitch skeletal muscles

Mammalian cardiac muscle contains two myosin alkali light chains: 1) the atrial light chain (MLC1A), and 2) the ventricular light chain (MLC1V) predominantly expressed either in the atrium or in the ventricle. In this report we describe the isolation and characterization of the complete gene for rat MLC1V. The rat MLC1V gene is approximately 6.5 kb long and the mRNA coding sequences are organized in 7 different exons. Comparison of this gene sequence with other known MLC1 gene sequences revealed that the exon-intron organization is highly conserved within the MLC1 gene family. The derived protein sequence of rat MLC1V showed a higher sequence homology with human ventricular (96%) MLC1V than with rat fast skeletal MLC1f (74%), suggesting functional similarities between different MLC1V proteins. S1 nuclease mapping and primer extension analysis demonstrated that this gene is expressed only in ventricular and slow twitch skeletal muscle tissues and is transcribed from the same promoter and transcription initiation site.

Functional characterization of skeletal F-actin labeled on the NH2-terminal segment of residues 1-28

Rabbit skeletal alpha-actin was covalently labeled in the filamentous state by the fluorescent nucleophile, N-(5-sulfo-1-naphthyl)ethylenediamine (EDANS) in the presence of the carboxyl group activator 1-(3-dimethyl-aminopropyl)-3-ethylcarbodiimide (EDC). The coupling reaction was continued until the incorporation of nearly 1 mol EDANS/mol actin. After limited proteolytic digestion of the labeled protein and chromatographic identification of the EDANS-peptides, about 80% of the attached fluorophore was found on the actin segment of residues 1-28, most probably within the N-terminal acidic region of residues 1-7. A minor labeling site was located on the segment that consists of residues 40-113. No label was incorporated into the COOH-terminal moiety consisting of residues 113-375. The isolated EDANS-G-actin undergoes polymerization in the presence of salts but at a rate significantly greater than unlabeled actin. The EDANS-F-actin could be complexed to skeletal chymotryptic myosin subfragment 1 (S-1) and to tropomyosin. The complex formed between EDANS-F-actin and S-1 could not be further crosslinked by EDC but the two proteins were readily joined by glutaraldehyde as observed for native actin-S-1, suggesting that the EDANS-substituted carboxyl site is also involved in the EDC crosslinking of native actin to S-1. Moreover, the EDANS labeling of F-actin resulted in a 20-fold increase in the Km of the actin-activated Mg2+.ATPase of S-1. Thus, this labeling, while it did not much affect the rigor actin-S-1 interaction, changes the actin binding to the S-1-nucleotide complexes significantly. The selective introduction of a variety of spectral probes, like EDANS, or other classes of fluorophores, on the N-terminal region of actin, through the reported carbodiimide coupling reaction, would provide several different derivatives valuable for assessing the functional role of the negatively charged N-terminus of actin during its interaction with myosin and other actin-binding proteins.

A novel human myosin alkali light chain is developmentally regulated. Expression in fetal cardiac and skeletal muscle and in adult atria

We have isolated cDNA recombinant phages encoding the embryonic isoform of the myosin alkali light chain (MLC1emb) from a human fetal skeletal muscle library. The cDNA clones were detected by their weak cross-hybridization to a human MLC1F and MLC3F cDNA clone. Nucleotide sequence analysis of the complete cDNA (GT14) revealed an open reading frame for 197 amino acids. The derived protein sequence constitutes the first structural information on this myosin isoform of any organism. Remarkable structural similarities to other alkali MLC polypeptides, particularly to those of the slow-muscle type, are evident. Under conditions of high stringency, the GT14 clone hybridized to an abundant mRNA species in fetal ventricular muscle and adult atrial muscle, whereas in fetal skeletal muscle only a very weakly hybridizing mRNA component was detected. These mRNAs were indistinguishable by size and the thermal stability of their hybrids formed with the DNA insert of clone GT14. We therefore conclude that identical mRNA is expressed in these tissues, presumably transcribed from the same gene. According to its pattern of mRNA expression, the novel MLC isoform described here was designated as "embryonic and atrial myosin light chain" (MLC1emb/A) in reference to its developmental stage-specific and tissue-specific appearance in embryonic skeletal muscle, fetal ventricle and adult atrium.

Cross-linking of the skeletal myosin subfragment 1 heavy chain to the N-terminal actin segment of residues 40-113

Glutaraldehyde (GA) and N-(ethoxycarbonyl)-2-ethoxy-1,2-dihydroquinoline (EEDQ), a hydrophobic, carboxyl group directed, zero-length protein cross-linker, were employed for the chemical cross-linking of the rigor complex between F-actin and the skeletal myosin S-1. The enzymatic properties and structure of the new covalent complexes obtained with both reagents were determined and compared to those known for the EDC-acto-S-1 complex. The GA- or EEDQ-catalyzed covalent attachment of F-actin to the S-1 heavy chain induced an elevated Mg2+-ATPase activity. The turnover rates of the isolated cross-linked complexes were similar to those for EDC-acto-S-1 (30 s-1). The solution stability of the new complexes is also comparable to that exhibited by EDC-acto-S-1. The proteolytic digestion of the isolated AEDANS-labeled covalent complexes and direct cross-linking experiments between actin and various preformed proteolytic S-1 derivatives indicated that, as observed with EDC, the COOH-terminal 20K and the central 50K heavy chain fragments are involved in the cross-linking reactions of GA and EEDQ. KI-depolymerized acto-S-1 complexes cross-linked by EDC, GA, or EEDQ were digested by thrombin which cuts only actin, releasing S-1 heavy chain-actin peptide cross-linked complexes migrating on acrylamide gels with Mr 100K (EDC), 110K and 105K (GA), and 102K (EEDQ); these were fluorescent only when fluorescent S-1 was used. They were identified by immunostaining with specific antibodies directed against selected parts of he NH2-terminal actin segment of residues 1-113.(ABSTRACT TRUNCATED AT 250 WORDS)