Amino acid sequence of the 203-residue fragment of the heavy chain of chicken gizzard myosin containing the SH1-type cysteine residue

Thiol reactivity as a sensor of rotation of the converter in myosin

Smooth muscle myosin has two reactive thiols located near the C-terminal region of its motor domain, the "converter", which rotates by approximately 70 degrees upon the transition from the "nucleotide-free" state to the "pre-power stroke" state. The incorporation rates of a thiol reagent, 5-(((2-iodoacetyl)amino)ethyl)aminonaphthalene-1-sulfonic acid (IAEDANS), into these thiols were greatly altered by adding ATP or changing the myosin conformation. Comparisons of the myosin structures in the pre-power stroke state and the nucleotide-free state explained why the reactivity of both thiols is especially sensitive to a conformational change around the converter, and thus can be used as a sensor of the rotation of the converter. Modeling of the myosin structure in the pre-power stroke state, in which the most reactive thiol, "SH1", was selectively modified with IAEDANS, revealed that this label becomes an obstacle when the converter completely rotates toward its position in the pre-power stroke state, thus resulting in incomplete rotation of the converter. Therefore, we suggest that the limitation of the converter rotation by modification causes the as-yet unexplained phenomena of SH1-modified myosin, including the inhibition of 10S myosin formation and the losses in phosphorylation-dependent regulation of the basic and actin-activated Mg-ATPase activities of myosin.

Interaction between the heavy and the regulatory light chains in smooth muscle myosin subfragment 1

The interaction between the heavy and the regulatory light chains within chicken gizzard myosin heads was investigated by using a zero-length chemical cross-linker, 1-ethyl-3-[3-(dimethylamino)-propyl]carbodiimide (EDC). The chicken gizzard subfragment 1 (S-1) used was treated with papain so that the heavy chain was partly cleaved into the NH2-terminal 72K and the COOH-terminal 24K fragments and the regulatory light chain into the 16K fragment. S-1 was reacted with EDC either alone or in the presence of ATP or F-actin. In all cases, the 16K fragment of the regulatory light chain formed a covalent cross-link with the 24K heavy chain fragment but not with the 72K fragment. The 38K cross-linked peptide, which was the product of cross-linking between the 16K light chain and the 24K heavy chain fragments, was isolated and further cleaved with cyanogen bromide and arginylendopeptidase. Smaller cross-linked peptides were purified by reverse-phase HPLC and then characterized by amino acid analysis and sequencing. The results indicated that cross-linking occurred between Lys-845 in the heavy chain and Asp-168, Asp-170, or Asp-171 in the regulatory light chain. The position of the cross-linked lysine was only three amino acid residues away from the invariant proline residue mapped as the S-1-rod hinge by McLachlan and Karn [McLachlan, A. D., & Karn, J. (1982) Nature (London) 299, 226-231]. We propose that the COOH-terminal region of the regulatory light chain is located in the neck region of myosin and that this region and the phosphorylation site of the regulatory light chain together may play a role in the phosphorylation-induced conformational change of gizzard myosin.

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.

Reaction of thiol groups of gizzard myosin heavy chains with 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole

Chicken gizzard myosin treated with 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) resulted in a 65% inhibition of the K(+)-ATPase (myosin ATP phosphohydrolase (actin translocating), EC 3.6.1.32) activity and 3.5 mol of the reagent was bound per 4.7 x 10(5) g protein. The labeling was limited to the heavy chain region and none of the light chains were lost. MgATP had no effect on the inactivation or labeling pattern. Thiolysis of NBD-myosin with dithiothreitol restored the K(+)-ATPase activity and concurrently, 1 mol of the NBD group was removed from the heavy chain region. Cysteine residues were modified in NBD-myosin at sites other than the active site when the enzyme activity was inhibited. There was a difference in the extent of NBD-Cl modification of gizzard myosin at 0.6 M KCl (6 S elongated state) when compared to that at 0.15 M KCl (10 S folded state). This was also seen in the heavy meromyosin-like chymotryptic fragments and tryptic fragments of NBD-myosin. The reagent NBD-Cl can detect changes in the conformation of gizzard myosin by way of its reaction with thiol groups of the heavy chain region.

A second consensus sequence of ATP-requiring proteins resides in the 21-kDa C-terminal segment of myosin subfragment 1

Previous comparisons of sequence homologies of ATP-requiring enzymes have defined three consensus sequences which appear to be involved in the binding of the nucleotide. One of these was identified in the N-terminal 27-kDa segment of the myosin heavy chain but the other two sequences have not hitherto been located in myosin. The present paper proposes that one of these other two consensus sequences is in the 21-kDa C-terminal portion of S1 and that it may contribute to the ATP binding domain.

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.

Glutamic acid-88 is close to SH-1 in the tertiary structure of myosin subfragment 1

The thiol-specific photoactivatable reagent benzophenone iodoacetamide (BPIA) can be selectively incorporated into the most reactive thiol, SH-1, of myosin S1, and upon photolysis, an intramolecular cross-link is formed between SH-1 and the N-terminal 25-kDa region of S1. If a Mg2+-nucleotide is present during photolysis, cross-links can be formed either with the 25-kDa region or with the central 50-kDa region [Lu et al. (1986) Proc. Natl. Acad. Sci. U.S.A. 83, 6392]. Comparison of the peptide maps of cross-linked and un-cross-linked S1 heavy chains indicates that the segment located about 12-16 kDa from the N-terminus of the heavy chain can be cross-linked to SH-1 via BPIA independently of the presence of a nucleotide whereas the segment located 57-60 kDa from the N-terminus can be cross-linked to SH-1 only in the presence of a Mg2+-nucleotide [Sutoh & Lu (1987) Biochemistry 26, 4511]. In this report, S1 was labeled with radioactive BPIA, photolyzed in the absence of nucleotide, and then degraded with proteolytic enzymes. Peptides containing cross-links were isolated by liquid chromatography and subjected to amino acid sequence analyses. The results show that Glu-88 is the major site and Asp-89 and Met-92 are the minor sites involved in cross-linking with SH-1 (Cys-707) via BPIA. These residues are very near the reactive lysine residue (Lys-83) but relatively remote in the primary structure from the putative nucleotide binding region.

Correlation among the proton charges and molecular masses of myosin subunits

The molecular masses and isoelectric points of myosin light and heavy chains were calculated from their known primary sequences and their respective distribution in a two-dimensional graph is displayed. Implications for the electrophoretic study of myosin subunits are inferred from this analysis.

Evidence for the association between two myosin heads in rigor acto-smooth muscle heavy meromyosin

The rigor complexes that formed between rabbit skeletal muscle F-actin and chicken gizzard heavy meromyosin (HMM), in which the heavy chains had been cleaved with trypsin into 24K, 50K, and 68K fragments, were examined by using the zero-length chemical cross-linker 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide (EDC). Two cross-linked products of approximate Mr 115K and 60K were generated. These products were not obtained by EDC treatment of HMM in the absence of F-actin. The HMM fragments that participated in cross-linking were identified by fluorescent labeling and amino acid composition studies. The 115K peptide was determined to be a covalently cross-linked complex that formed between actin and the COOH-terminal 68K fragment of the HMM heavy chain. Our results are in agreement with a previous study which proposed that the site of cross-linking between HMM and F-actin resides within the COOH-terminal 22K fragment of the myosin subfragment 1 heavy chain [Marianne-Pépin, T., Mornet, D., Bertrand, R., Labbé, J.-P., & Kassab, R. (1985) Biochemistry 24, 3024-3029]. The 60K peptide, however, was not a product of cross-linking between HMM and F-actin. On the basis of its amino acid composition, we concluded that this 60K peptide was a cross-linked dimer of the NH2-terminal 24K fragments of the HMM heavy chain. The cross-linking of acto-gizzard HMM significantly increased the Mg-ATPase activity of gizzard HMM without any observable phosphorylation of the regulatory (20K) light chains.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Complete primary structure of vertebrate smooth muscle myosin heavy chain deduced from its complementary DNA sequence. Implications on topography and function of myosin

The 1979 amino acid sequence of embryonic chicken gizzard smooth muscle myosin heavy chain (MHC) have been determined by cloning and sequencing its cDNA. Genomic Southern analysis and Northern analysis with the cDNA sequence show that gizzard MHC is encoded by a single-copy gene, and this gene is expressed in the gizzard and aorta. The encoded protein has a calculated Mr of 229 X 10(3), and can be divided into a long alpha-helical rod and a globular head. Only 32 to 33% of the amino acid residues in the rod and 48 to 49% in the head are conserved when compared with nematode or vertebrate sarcomeric MHC sequences. However, the seven residue hydrophobic periodicity, together with the 28 and 196 residue repeat of charge distribution previously described in nematode myosin rod, are all present in the gizzard myosin rod. Two of the trypsin-sensitive sites in gizzard light meromyosin have been mapped by partial peptide sequencing to 99 nm and 60 nm from the tip of the myosin tail, where these sites coincide with the two "hinges" for the 6 S/10 S transition. In the head sequence, several polypeptide segments, including the regions around the putative ATP-binding site and the reactive thiol groups, are highly conserved. These areas presumably reflect conserved structural elements important for the function of myosin. A multi-domain folding model of myosin head is proposed on the basis of the conserved sequences, information on the topography of myosin in the literature, and the predicted secondary structures. In this model, Mg2+ ATP is bound to a pocket between two opposing alpha/beta domains, while actin undergoes electrostatic interactions with lysine-rich surface loops on two other domains. The actin-myosin interactions are thought to be modulated through relative movements of the domains induced by the binding of ATP.