Comparative structure of the protease-sensitive regions of the subfragment-1 heavy chain from smooth and skeletal myosins

Filament evanescence of myosin II and smooth muscle function

Smooth muscle is an integral part of hollow organs. Many of them are constantly subjected to mechanical forces that alter organ shape and modify the properties of smooth muscle. To understand the molecular mechanisms underlying smooth muscle function in its dynamic mechanical environment, a new paradigm has emerged that depicts evanescence of myosin filaments as a key mechanism for the muscle’s adaptation to external forces in order to maintain optimal contractility. Unlike the bipolar myosin filaments of striated muscle, the side-polar filaments of smooth muscle appear to be less stable, capable of changing their lengths through polymerization and depolymerization (i.e., evanescence). In this review, we summarize accumulated knowledge on the structure and mechanism of filament formation of myosin II and on the influence of ionic strength, pH, ATP, myosin regulatory light chain phosphorylation, and mechanical perturbation on myosin filament stability. We discuss the scenario of intracellular pools of monomeric and filamentous myosin, length distribution of myosin filaments, and the regulatory mechanisms of filament lability in contraction and relaxation of smooth muscle. Based on recent findings, we suggest that filament evanescence is one of the fundamental mechanisms underlying smooth muscle’s ability to adapt to the external environment and maintain optimal function. Finally, we briefly discuss how increased ROCK protein expression in asthma may lead to altered myosin filament stability, which may explain the lack of deep-inspiration–induced bronchodilation and bronchoprotection in asthma.

Both heads of tissue-derived smooth muscle heavy meromyosin bind to actin in the presence of ADP

The effect of ADP and phosphorylation upon the actin binding properties of heavy meromyosin was investigated using three fluorescence methods that monitor the number of heavy meromyosin heads that bind to pyrene-actin: (i) amplitudes of ATP-induced dissociation, (ii) amplitudes of ADP-induced dissociation of the pyrene-actin-heavy meromyosin complex, and (iii) amplitudes of the association of heavy meromyosin with pyrene-actin. Both heads bound to pyrene-actin, irrespective of regulatory light chain phosphorylation or the presence of ADP. This behavior was found for native regulated heavy meromyosin prepared by proteolytic digestion of chicken gizzard myosin with between 5 and 95% heavy chain cleavage at the actin-binding loop, showing that two-head binding is a property of heavy meromyosin with uncleaved heavy chains. These data are in contrast to a previous study using an uncleaved expressed preparation (Berger, C. E., Fagnant, P. M., Heizmann, S., Trybus, K. M., and Geeves, M. A. (2001) J. Biol. Chem. 276, 23240-23245), which showed that one head of the unphosphorylated heavy meromyosin-ADP complex bound to actin and that the partner head either did not bind or bound weakly. Possible explanations for the differences between the two studies are discussed. We have shown that unphosphorylated heavy meromyosin appears to adopt a special state in the presence of ADP based upon analysis of actin-heavy meromyosin association rate constants. Data were consistent with one head binding rapidly and the second head binding more slowly in the presence of ADP. Both heads bound to actin at the same rate for all other states.

Differential scanning calorimetric study of myosin subfragment 1 with tryptic cleavage at the N-terminal region of the heavy chain

The thermal unfolding of myosin subfragment 1 (S1) cleaved by trypsin was studied by differential scanning calorimetry. In the absence of nucleotides, trypsin splits the S1 heavy chain into three fragments (25, 50, and 20 kDa). This cleavage has no appreciable influence on the thermal unfolding of S1 examined in the presence of ADP, in the ternary complexes of S1 with ADP and phosphate analogs, such as orthovanadate (Vi) or beryllium fluoride (BeFx), and in the presence of F-actin. In the presence of ATP and in the complexes S1.ADP.Vi or S1.ADP.BeFx, trypsin produces two additional cleavages in the S1 heavy chain: a faster cleavage in the N-terminal region between Arg23 and Ile24, and a slower cleavage at the 50 kDa fragment. It has been shown that the N-terminal cleavage strongly decreases the thermal stability of S1 by shifting the maximum of its thermal transition by about 7 degrees C to a lower temperature, from 50 degrees C to 42.4 degrees C, whereas the cleavage at both these sites causes dramatic destabilization of the S1 molecule leading to total loss of its thermal transition. Our results show that S1 with ATP-induced N-terminal cleavage is able, like uncleaved S1, to undergo global structural changes in forming the stable ternary complexes with ADP and Pi analogs (Vi, BeFx). These changes are reflected in a pronounced increase of S1 thermal stability. However, S1 cleaved by trypsin in the N-terminal region is unable, unlike S1, to undergo structural changes induced by interaction with F-actin that are expressed in a 4-5 degrees C shift of the S1 thermal transition to higher temperature. Thus, the cleavage between Arg23 and Ile24 does not significantly affect nucleotide-induced structural changes in the S1, but it prevents structural changes that occur when S1 is bound to F-actin. The results suggest that the N-terminal region of the S1 heavy chain plays an important role in structural stabilization of the entire motor domain of the myosin head, and a long-distance communication pathway may exist between this region and the actin-binding sites.

Differences in the ionic interaction of actin with the motor domains of nonmuscle and muscle myosin II

Changes in the actin-myosin interface are thought to play an important role in microfilament-linked cellular movements. In this study, we compared the actin binding properties of the motor domain of Dictyostelium discoideum (M765) and rabbit skeletal muscle myosin subfragment-1 (S1). The Dictyostelium motor domain resembles S1(A2) (S1 carrying the A2 light chain) in its interaction with G-actin. Similar to S1(A2), none of the Dictyostelium motor domain constructs induced G-actin polymerization. The affinity of monomeric actin (G-actin) was 20-fold lower for M765 than for S1(A2) but increasing the number of positive charges in the loop 2 region of the D. discoideum motor domain (residues 613-623) resulted in equivalent affinities of G-actin for M765 and for S1. Proteolytic cleavage and cross-linking approaches were used to show that M765, like S1, interacts via the loop 2 region with filamentous actin (F-actin). For both types of myosin, F-actin prevents trypsin cleavage in the loop 2 region and F-actin segment 1-28 can be cross-linked to loop 2 residues by a carbodiimide-induced reaction. In contrast with the S1, loop residues 559-565 of D. discoideum myosin was not cross-linked to F-actin, probably due to the lower number of positive charges. These results confirm the importance of the loop 2 region of myosin for the interaction with both G-actin and F-actin, regardless of the source of myosin. The differences observed in the way in which M765 and S1 interact with actin may be linked to more general differences in the structure of the actomyosin interface of muscle and nonmuscle myosins.

Baculovirus expression of chicken nonmuscle heavy meromyosin II-B. Characterization of alternatively spliced isoforms

We have expressed two truncated isoforms of chicken nonmuscle myosin II-B using the baculovirus expression system. One of the expressed heavy meromyosins (HMMexp) consists of two 150-kDa myosin heavy chains (MHCs), comprising amino acids 1-1231 as well as two pairs of 20-kDa and 17-kDa myosin light chains (MLCs) in a 1:1:1 molar ratio. The second HMMexp was identical except that it contained an insert of 10 amino acids (PESPKPVKHQ) at the 25-50-kDa domain boundary in the subfragment-1 region of the MHC. These 10 amino acids include a consensus sequence (SPK) for proline-directed kinases. Expressed HMMs were soluble at low ionic strength and bound to rabbit skeletal muscle actin in an ATP-dependent manner. These properties afforded a rapid purification of milligram quantities of expressed protein. Both isoforms were capable of moving actin filaments in an in vitro motility assay and manifested a greater than 20-fold activation of actin-activated MgATPase activity following phosphorylation of the 20-kDa MLC. HMMexp with the 10-amino acid insert was phosphorylated by Cdc2, Cdk5, and mitogen-activated protein kinase in vitro to 0.3-0.4 mol of PO4/mol of MHC. The site phosphorylated in the MHC was identified as the serine residue present in the 10-amino acid insert and its presence was confirmed in bovine brain MHCs. Characterization of the baculovirus expressed noninserted and inserted MHC isoforms with respect to actin-activated MgATPase activity and ability to translocate actin filaments in an in vitro motility assay produced the following average values following MLC phosphorylation: noninserted HMMexp, Vmax = 0.28 s-1, Km = 12.7 microM; translocation rate = 0.077 micron/s; inserted HMMexp, Vmax = 0.37 s-1, Km = 15.1 microM; translocation rate = 0.092 micron/s.

Polyacrylamide gel electrophoretic methods in the separation of structural muscle proteins

Polyacrylamide gel electrophoresis plays a major role in analyzing the function of muscle structural proteins. This review describes one- and two-dimensional gel electrophoretic methods for qualitative and quantitative investigation of the muscle proteins, with special emphasis on determination of protein phosphorylation. The electrophoretic studies established the subunit structures of the muscle proteins, characterized their multiple forms, revealed changes in subunit composition or shifts in isoform distribution of specific proteins during development, upon stimulation or denervation of the muscle. Protein phosphorylation during muscle contraction is preferentially studied by two-dimensional gel electrophoresis. The same method demonstrated protein alterations in human neuromuscular diseases.

The yeast type II myosin heavy chain: analysis of its predicted polypeptide sequence

We have completed the nucleotide sequence of the yeast MYO1 gene and deduced its amino acid sequence. The gene is 5553 bp long and contains no introns. Analysis of the sequence, as well as its comparison with other myosins, demonstrate that the yeast protein is a type II myosin heavy chain with characteristic head and tail regions. The latter domain contains six proline residues in two clusters of three, at approximately two thirds from the start of the gene.

Conformational transitions within the head and at the head-rod junction in smooth muscle myosin studied with a limited proteolysis method

It was previously shown that tryptic digestion of subfragment 1 (S1) of skeletal muscle myosins at 0 degree C results in cleavage of the heavy chain at a specific site located 5 kDa from the NH2-terminus. This cleavage is enhanced by nucleotides and suppressed by actin and does not occur at 25 degrees C, except in the presence of nucleotide. Here we show a similar temperature sensitivity and protection by actin of an analogous chymotryptic cleavage site in the heavy chain of gizzard S1. The results support the view that the myosin head, in general, can exist in two different conformational states even in the absence of nucleotides and actin, and indicate that the heavy chain region 5 kDa from the NH2-terminus is involved in the communication between the sites of nucleotide and actin binding. We also show here for the first time that the S1-S2 junction in gizzard myosin can be cleaved by chymotrypsin and that this cleavage (observed in papain-produced S1 devoid of the regulatory light chain) is also temperature-dependent but insensitive to nucleotides and actin. It is suggested that the temperature-dependent alteration in the flexibility of the head-rod junction, which is apparent from these and similar observations on skeletal muscle myosin [Miller, L. & Reisler, E. (1985) J. Mol. Biol. 182, 271-279; Redowicz, M.J. & Strzelecka-Gołaszewska, H. (1988) Eur. J. Biochem. 177, 615-624], may contribute to the temperature dependence of some steps in the cross-bridge cycle.

New subfragment 1 of skeletal muscle myosin obtained by thrombin cleavage

The head of the myosin molecule (i.e., subfragment 1 with a heavy chain of 95 kDa) is usually obtained by chymotryptic cleavage in the presence of a divalent cation chelator. In the present work, we used another specific proteolytic enzyme, thrombin, to produce a limited cut within the myosin molecule, resulting in a new species of N-terminal fragment. Treatment of skeletal muscle myosin yielded a 97-kDa split heavy chain associated with intact light chains, corresponding to a single cut. The ATPase activities of this new S-1 derivative were slightly affected by the breakdown. It recognized actin in an ATP-dependent manner, as expected, with an affinity 2-5 times higher than that of the usual chymotryptic S-1 preparation but with a very different electron microscopic pattern. Functional differences are noted, and we involve them more precisely in relation to possible structural aspects of the additional C-terminal segment extending the usual S-1 heavy chain from 95 to 97 kDa.

ATP-induced structural change in myosin subfragment-1 revealed by the location of protease cleavage sites on the primary structure

To understand the nature of the ATP-induced structural change in myosin subfragment-1, rabbit and chicken skeletal subfragments-1s were cleaved by various proteolytic enzymes in the absence, and in the presence, of ATP and the exact locations of the cleavage sites that were affected by ATP were determined from the amino end analysis of fragments by the use of a protein sequencer. It was found that subtilisin cleaved a site between Gln27 and Asn28 of rabbit subfragment-1 and between Gln28 and Asn29 of chicken subfragment-1 only in the presence of ATP. Thermolysin cleaved a site between Pro31 and Phe32 of chicken subfragment-1 in the presence of ATP, but the same site of rabbit subfragment-1 was not cleaved. The location of these sites is quite similar to the ATP-induced chymotryptic cleavage site of chicken gizzard heavy meromyosin, between Trp29 and Ser30 as reported by others. It is suggested, therefore, that the structure and the ATP-induced structural change in the regions are similar in these subfragment-1s. ATP also changes the cleavage rate of the 26K-50K junction by many proteases. Exact cleavage sites were determined and the relationship between their location and the suppression or the enhancement by ATP of the cleavage was studied. It was found that the cleavage sites were restricted to a quite narrow region and only the cleavage by thermolysin that attacked the middle of the region was enhanced by ATP. The distribution of the cleavage sites and the effect of ATP suggest that ATP induces drastic structural change at the middle of the 26K-50K junction region. The region attacked easily by many proteases coincided very well with a hydrophilic region indicated by the hydropathy index. The region probably protrudes outside and is, therefore, easily attacked by many proteases.

Selective cleavage at lysine of the 50 kDa-20 kDa connector loop segment of skeletal myosin S-1 by endoproteinase Arg-C

The reaction of endoproteinase Arg-C on the skeletal myosin head heavy chain was investigated through characterization of peptides and amino acid sequence analysis. The protease splits exclusively the 50 kDa-20 kDa junction at the lysine cluster spanning residues 639-641 and does not affect any other protease-sensitive region of the entire myosin heavy chain. The sensitivity of the cleavage to actin and nucleotide binding makes this protease a very specific conformational probe of S-1. The nicked S-1 derivative, containing an intact NH2-terminal 75 kDa fragment, may serve as a tool for gaining further insights into the domain structure and function of the myosin head.

Carbodiimide-catalyzed cross-linking sites in the heads of gizzard heavy meromyosin attached to F-actin

In the rigor complex between rabbit skeletal muscle F-actin and chicken gizzard heavy meromyosin (HMM), the direct contact between two HMM heads was demonstrated by using a zero-length cross-linker 1-ethyl-3-[3-(dimethylamino)propyl]maleimide (EDC) [Onishi, H., Maita, T., Matsuda, G., & Fujiwara, K. (1989) Biochemistry (preceding paper in this issue)]. Here, the 60K peptide which was a product of the EDC cross-linking between two 24K heavy chain (tryptic) fragments of HMM was further fragmented with cyanogen bromide, and the location of the cross-linking sites on the amino acid sequence of the HMM heavy chain was investigated. The result showed that one site resided within the 77-residue peptide region (residues 1-77) on one head of HMM, whereas the other site belonged to the 40-residue peptide region (residues 164-203) on the other head. This finding suggests that the two HMM heads are in contact with each other at different sites. Ultracentrifugal fractionation revealed that the head-to-head cross-linked gizzard HMM could be reversibly released from F-actin in the presence of Mg-ATP. The yield of the head-to-head cross-linking was not significantly changed with the acto-HMM complex between actin/HMM head molar ratios of 1 and 4, and it was very slightly decreased even at a molar ratio of 8, where HMM molecules were attached sparsely to actin filaments.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional sequences of the myosin head

Muscle contraction originates from the sliding of myosin filaments on actin filaments, the energy for which is supplied by the hydrolysis of adenosine-5-triphosphate (ATP) by myosin. The nucleotide first binds to the acto-myosin complex in the myosin head (or subfragment-1), producing a conformational change which induces actin dissociation. The release of phosphate (Pi) then allows a return to the strong actin-myosin association, corresponding to the rigor state. We discuss here certain controversial points arising from current concepts of the actin and nucleotide binding regions at the amino acid sequence level within the subfragment-1 heavy chain. We consider the actin and nucleotide binding regions to be two distinct sites (for each of these regions) one of which is shared competitively between actin and the nucleotide. In our model the cyclical actin-S1 association-dissociation steps correspond to different ATP, actin and ADP affinities for the same amino acid sequence of the S1 heavy chain, contributing alternatively to a single hydrolytic nucleotide site or a strong actin site. We propose the existence of a flexible segment that forms or dismantles the nucleotide or actin sites. The large region (amino acids 540-707) overlapping the actin-myosin interface appears to be the main flexible region of the S1 molecule and we propose that this particular sequence plays a key role in the dissociation pathway of the actin-myosin complex and in the conversion of chemical energy into the mechanical energy of contraction.

A 35-kilodalton fragment from gizzard smooth muscle caldesmon that induces F-actin bundles

Specific thrombin proteolysis of native 120-kDa gizzard caldesmon gave rise to a major cleavage into an N-terminal 90-kDa and a C-terminal 35-kDa fragment. Fluorescent labeling, cosedimentation, passage through an affinity column, and carbodiimide crosslinking with actin revealed that the 35-kDa purified segment of the molecule contains the actin and the calcium-calmodulin binding regions. Electron microscopic analysis of its actin complex demonstrated that the 35-kDa segment possesses the bundling properties of the intact molecule. Thus, a possible pathway for the expression of the caldesmon regulatory function during smooth muscle contraction would be a conformational change twisting the helicoïdal structure of the actin filament, which occurs when the 35-kDa caldesmon portion binds to it.

Identification of a 15 kilodalton actin binding region on gizzard caldesmon probed by chemical cross-linking

Fluorescent labeling, limited proteolysis, amino acid sequence determinations, affinity chromatography and specific chemical crosslinking were used to determine the smallest fragment of gizzard caldesmon that interacts with actin. The time course of cleavage with thrombin or submaxillaris arginase-C protease indicates that 90kDa and 35kDa fragments are the two major pieces of the 120kDa native protein. Amino acid sequence determination indicates that the 90kDa fragment is the N-terminal portion of the molecule. Further degradation gave rise to a 15kDa product whose N-terminal amino acid sequence was determined within the first 28 amino acids. Carbodiimide crosslinking with actin revealed that the 15kDa part of the molecule is probably not involved in the actin binding process but may participate in a twisting of the F-actin filament and be responsible of the caldesmon regulatory function during smooth muscle contraction.

The internal crosslinking of the S1 heavy chain from smooth muscle probed by dibromobimane

The reaction of the crosslinker dibromobimane has recently revealed a functionally important internal loop structure within the skeletal myosin S1 heavy chain where Cys-522 of the 50K domain and Cys-707 (SH1) of the 20K region are spatially juxtaposed. Here we have studied the possible relevance of this topological feature to the architecture and transducing activity of the myosin head in general, by extending the dibromobimane modification to smooth muscle myosin. Treatment of chicken gizzard myosin S1 with dibromobimane resulted in intramolecular crosslinking between the C-terminal 25K and central 50K segments of the S1 heavy chain. The data suggest a conservation at the 50K-25K interface of smooth muscle S1 heavy chain and the importance of the neighboring SH1 region, whose conformation may play an important role in energy transduction by the myosin head.