Electron microscopic study on the location of 23 kDa and 50 kDa fragments in skeletal myosin head

How are myosin fragments bound to nitrocellulose film?

In an attempt to identify the binding sites of myosin fragments to the nitrocellulose film in a flow cell, proteolytic digestion was performed of myosin fragments bound to the substrate. Chymotryptic digestion of papain.Mg-subfragment-1 (S1) showed that a large proportion of S1 were bound near the tail and that the digestion site was protected for some S1. The binding of S1 near the tail was also confirmed by the production of extra long subfragment-2 (S2) by chymotryptic digestion of heavy meromyosin (HMM). Papain digestion of HMM released slightly smaller HMM from the substrate and impaired the motility at the same time. Hence, the motility of HMM is supported largely by those bound to the substrate near the C terminus. These results demonstrate the importance of flexible regions of myosin in generating active movements.

Expression of human beta-myosin heavy chain fragments in Escherichia coli; localization of actin interfaces on cardiac myosin

A cDNA clone coding for an internal fragment of slow-cardiac beta-myosin heavy chain was isolated from a lambda gt10 human skeletal muscle library. Six overlapping cDNA subclones, which span myosin heavy chain subregions and presumably interact with actin, were derived from this clone, fused to a beta-galactosidase vector and expressed in Escherichia coli. Three of the subclones were obtained by PCR (polymerase chain reaction) which enables gene or cDNA fragments to be amplified independently of preexisting restriction sites. Initially, various experiments were carried out using a long MHC (myosin heavy chain) fusion protein containing the 50 kDa-20 kDa connecting region, the whole 20 kDa region and the short subfragment 2 region. This MHC fusion protein was chemically or proteolytically cleaved in the same conditions as the native myosin molecule. Whole and truncated forms of the MHC fusion protein were separated on polyacrylamide gels, electroblotted on nitrocellulose sheets and renatured. They were then assayed in overlay experiments with F-actin and/or myosin light chains in solution. Specific antibodies were used to detect interactions between heavy chain fragments and F-actin or light chains. We thus observed that one long heavy chain fragment synthesized by E. coli behaved like proteolytic or chemical MHC preparations made from native myosin molecules. Two chymotryptic fragments of the MHC fusion protein, which are soluble at low ionic strength, cosedimented with F-actin in solution. Our results demonstrate that, in actin overlay experiments with whole fusion proteins, interactions seem to be due to the heavy chain fragment, not to the bacterial component. All interactions were non ATP-sensitive. We further investigated the possible participation of the six recombinant MHC fragments in contributing to the actomyosin interfaces on the 50 kDa-20 kDa regions of the human cardiac beta-MHC. The present procedure, which enables the synthesis of any MHC fragment independent of any protease site, is a powerful new tool for studying structure-function relationships within the myosin molecule family.

Domain structure of myosin subfragment-1. Selective denaturation of the 50 kDa segment

The structure of the myosin subfragment-1 (S1) from rabbit skeletal muscle was studied using differential scanning microcalorimetry. Three independently melting regions (domains) were revealed in S1. Selective denaturation of the middle 50 kDa segment of the S1 heavy chain resulted in the disappearance of the heat sorption peak corresponding to the melting of the first, the most thermolabile domain without any effect on the thermally induced blue shift of the intrinsic tryptophan fluorescence spectrum which occurs within the temperature region of melting of the second domain. It is concluded that the most thermolabile domain seems to correspond to the N-terminal part of the 50 kDa segment devoid of tryptophan residues.

Bidirectional movement of actin filaments along tracks of myosin heads

It is well established that muscle contraction results from the relative sliding of actin and myosin filaments. Both filaments have definite polarities and well-ordered structures. Thick filaments, however, are not vital for supporting movement in vitro. Previously we have demonstrated that actin filaments can move continuously on myosin fragments (subfragment-1 or heavy meromyosin (HMM] that are bound to a nitrocellulose surface. Here we report that actin filaments can move in opposite directions on tracks of myosin heads formed when actin filaments decorated with HMM are placed on a nitrocellulose surface. The actin filaments always move forward, frequently changing the direction of the movement, but never move backward reversing the polarity of the movement. The direction of movement is therefore determined by the polarity of the actin filament. These results indicate that myosin heads have considerable flexibility.

On the origin and transmission of force in actomyosin subfragment 1

A proximity map showing the three-dimensional arrangement of 12 chemically defined points in actomyosin subfragment 1 is developed and roughly correlated with published electron microscope reconstruction of others. Several additional points and topological relationships in the primary polypeptide chain folding are assimilated into this model. Certain crosslinkings and distance change observations are interpreted as indicators of transmission of force/displacement between the nucleotide-binding and an actin-binding site--i.e., as indications of how energy is transduced in this system.

Electron microscopic mappings of myosin head with site-directed antibodies

Site-directed antibodies were raised against three synthetic peptides whose sequences correspond to a region around the reactive lysine residue and two protease-sensitive regions of subfragment 1 (S1) of skeletal muscle myosin (one at the junction of the 23,000 Mr and 50,000 Mr segments, the J1 junction; and the other at the junction of the 50,000 Mr and 20,000 Mr segments of the heavy chain, the J2 junction). The antisera cross-reacted with intact myosin with titres of 5 x 10(4) (anti-J1 antiserum) and 10(4) (anti-J2 and anti-reactive lysine residue antisera). Site-specific antibodies purified by S1-Sepharose readily bound to myosin. Electron microscopic examinations of antibody-myosin complexes revealed that the J1 and J2 junctions are located 15 nm and 16 nm from the head-rod junction, respectively, while the reactive lysine residue region is 13 nm from the junction.