Identification and characterization of Tetrahymena myosin

Myosin was partially purified from ciliated protozoan Tetrahymena pyriformis. Tetrahymena myosin has a fibrous tail with two globular heads at one end and contains 220-kDa heavy chains. The tail length of the molecule (200 nm) is longer than that of myosins from other animals (approximately 160 nm). A sample after HPLC column chromatography containing 220-kDa peptide showed a myosin-specific K⁺-/NH₄⁺-EDTA-ATPase activity. Polyclonal anti-crayfish myosin heavy chain antibody reacted with Tetrahymena 220-kDa myosin heavy chain, and monoclonal anti-pan myosin antibody reacted with Tetrahymena 180-kDa peptide. The isolated 180-kDa peptide was identified as a clathrin heavy chain.     

Identification of the serine residue phosphorylated by protein kinase C in vertebrate nonmuscle myosin heavy chains

Two-dimensional mapping of the tryptic phosphopeptides generated following in vitro protein kinase C phosphorylation of the myosin heavy chain isolated from human platelets and chicken intestinal epithelial cells shows a single radioactive peptide. These peptides were found to comigrate, suggesting that they were identical, and amino acid sequence analysis of the human platelet tryptic peptide yielded the sequence -Glu-Val-Ser-Ser(PO4)-Leu-Lys-. Inspection of the amino acid sequence for the chicken intestinal epithelial cell myosin heavy chain (196 kDa) derived from cDNA cloning showed that this peptide was identical with a tryptic peptide present near the carboxyl terminal of the predicted alpha-helix of the myosin rod. Although other vertebrate nonmuscle myosin heavy chains retain neighboring amino acid sequences as well as the serine residue phosphorylated by protein kinase C, this residue is notably absent in all vertebrate smooth muscle myosin heavy chains (both 204 and 200 kDa) sequenced to date.     

A new cardiac myosin characterized from the canine atria.

Canine atrial myosin light chains were electrophoretically distinct from myosins of canine ventricles on 5–20% polyacrylamide gradient slab gels (SDS), giving molecular weights of 26,000 and 21,000 as compared to 28,000 and 18,500 for ventricular myosin light chains. While atrial myosin heavy chains were immunologically identical with ventricular myosin heavy chains, in contrast, there was 8.0% relative cross-reactivity of atrial myosin light chains with left ventricular myosin light chains by radioimunoassay. According to charge separation on two-dimensional polyacrylamide urea gels, atrial myosin light chains were different from those of ventricular myosins. Variances in ATPase activities between atrial and ventricular myosins were strongly demonstrated. There was a lower K⁺ activated ATPase activity in atrial myosin, however the Ca²⁺ activated ATPase activity, at ATP saturation levels, was higher in atrial myosin as compared to ventricular myosins.     

Thyroid hormone stimulates synthesis of a cardiac myosin isozyme. Comparison of the two-two-dimensional electrophoretic patterns of the cyanogen bromide peptides of cardiac myosin heavy chains from euthyroid and thyrotoxic rabbits.

The CNBr peptides of [14C]carboxymethylated cardiac myosin heavy chains from euthyroid and thyrotoxic rabbits have been compared using a two-dimensional electrophoretic system. The results indicated that there were extensive differences in the peptide "maps" of these heavy chains, which included differences in the distribution of radiolabeled thiol peptides. Also, the patterns of heavy chain peptides from the cardiac myosins have been compared with those produced by the heavy chain myosin isozymes from skeletal muscles. Peptide maps of heavy chains from red skeletal muscle myosin closely resembled the pattern of peptides found with cardiac myosin heavy chains from euthyroid rabbits. However, peptide maps of heavy chains from white skeletal muscle myosin were dissimilar to those of the cardiac myosin isozymes. We conclude that thyroxine administration stimulates the synthesis of a cardiac myosin isozyme with a heavy chain primary structure which is different from either of the skeletal muscle myosin isozymes.     

The behavior of fetal canine cardiac cells in culture: synthesis and phosphorylation of myosin.

This study describes the first in vitro culturing of canine cardiac cells. Canine cardiac myosin which was synthesized in a 14-day tissue culture, based on l-[³H]leucine incorporation, was precipitated with goat γG antimyosin (cardiac-specific) and analyzed on dodecylsulfate gels; the specific activity of the highly purified myosin chains was determined. Incorporation of ³²PO₄ was similarly analyzed. The comparative degree of synthesis and phosphorylation of myosin chains, occurring in culture, was the same as that obtained in vivo. Both l-[³H]leucine and ³²PO₄ incorporation were inhibited by addition of cycloheximide to the culture medium. Removal of ³²PO₄ from myosin heavy chains with base treatment indicated the presence of phosphoserine and/or phosphothreonine in canine cardiac myosin heavy chains. Myosins from fetal and adult canine cardiac tissue were immunologically identical with each other and with the cultured fetal tissue; all had similar myosin ATPase activity and the degree of heavy chain phosphorylation was similar. The tissue and techniques used here gave a high yield of cardiac myocytes based principally on synthesis of cardiac-specific myosin.     

Comparison of ATPase activities and heavy chains of rabbit atrial and thyrotoxic ventricular myosin subfragment-1

Summary Subfragment-1 of rabbit atrial and thyrotoxic ventricular myosin (V₁ isomyosin) has been prepared and purified by DEAF-cellulose column chromatography. Pyrophosphate-polyacrylamide gel electrophoretic patterns and column chromatographic profile of the atrial subfragment differ from those of thyrotoxic ventricular myosin subfragment-1. On the other hand, Ca²⁺, Mg²⁺ and actin-activated ATPase activities of these subfragments are identical. Comparison of the peptide mapping by limited proteolysis in the presence of sodium dodecyl sulfate of the heavy and the light subunits of these subfragments reveals that the patterns for the heavy chain peptides of these subfragments are substantially similar but their light chain peptide patterns differ. The results suggest that the enzymatic and structural similarities that have been recognized between these isoenzymes using intact myosin hold true for the myosin subfragment-1.The differences between these subfragments are due to the differences in the light chains associated with them.Abbreviations EDTA Ethylene Diamine Tetra-acetic Acid - SDS Sodium Dodecyl Sulfate - S₁ myosin subfragment-1 - HC Heavy Chain - LC Light Chain     

Living macrophages phosphorylate the 20,000 dalton light chains and heavy chains of myosin

Brief incubation of rabbit alveolar macrophages in medium containing ³²Pi results in the incorporation of radioactivity into the 20 KD light chains and into the 220 KD heavy chains of myosin. Phosphorylation of the heavy chain is mediated by a kinase that is probably not myosin light chain kinase. Limited proteolysis of the phosphorylated myosin shows that radioactivity is associated with the rod portion of the heavy chain.     

Purification of messenger ribonucleic acids for fast and slow myosin heavy chains by indirect immunoprecipitation of polysomes from embryonic chick skeletal muscle

Fast and slow myosin heavy chain mRNAs were isolated by indirect immunoprecipitation of polysomes from 14-day-old embryonic chick leg muscle. The antibodies were prepared against myosin heavy chains purified by NaDod-SO4-polyacrylamide gel electrophoresis and were shown to be specific for fast and slow myosin heavy chains. The RNA fractions directed the synthesis of myosin heavy chains in a cell-free translation system from wheat germ. Several smaller peptides were also synthesized in lower concentrations. These probably are partial products of myosin heavy chains, since they are immunoprecipitated with antibodies to myosin heavy chains. Immunoprecipitation of the translation products with the antibodies to fast and slow myosin heavy chains showed the RNA preparations to be approximately 94% enriched for fast myosin heavy chain mRNA and approximately 84% enriched for slow myosin heavy chain mRNA with respect to myosin HC type. Peptides having slightly different mobilities on NaDodSO4-polyacrylamide gels were immunoprecipitated by antibodies to fast and slow myosin heavy chains.     

Myosin heavy chains from two different adult fast-twitch muscles have different peptide maps but identical mRNAs

Myosin heavy chains prepared from the pectoralis major and from the posterior latissimus dorsi of the same adult chicken exhibit different peptide maps when cleaved with Staphylococcus aureus V8 protease. These differences were observed at five different enzyme concentrations and in chickens of various strains. The cleavage pattern of pectoralis major myosin heavy chain from different adult chickens was always identical, as was that of posterior latissimus dorsi myosin heavy chain, demonstrating the reproducibility of the technique. However, when RNAs extracted from the pectoralis major and from the posterior latissimus dorsi were translated in a cell-free reticulocyte lysate, the myosin heavy chain encoded by pectoralis major RNA and the myosin heavy chain encoded by posterior latissimus dorsi RNA exhibited identical peptide maps. These results suggest that the different peptide maps of myosin heavy chains from the pectoralis major and posterior latissimus dorsi may arise from posttranslational modifications.     

The Two Myosin Isoenzymes of Chicken Anterior Latissimus dorsi Muscle Contain Different Myosin Heavy Chains Encoded by Separate mRNAs

Abstract. The two myosin isozymes (SM1 and SM2) of the anterior latissimus dorsi muscle of the chicken change in relative concentration during development. As SM1 decreases from 13 days of embryonic growth through 1 year of adult maturation, SM2 increases. In the adult muscle SM2 accounts for over 95% of the total myosin. The myosin heavy chains of the two isozymes are distinctly different and may be separated from each other by 5% SDS polyacrylamide gel electrophoresis. The faster migrating myosin heavy chain is identified as originating from SM1 and the slower migrating myosin heavy chain from SM2 myosin isozymes. The myosin heavy chains change in relative concentration during development exactly parallel with changes in SM1 and SM2 isozyme levels. Peptide map analysis also reveals that SM1 myosin heavy chains and SM2 myosin heavy chains are distinctly different. When RNA from the ALD muscle is added to reticulocyte lysate protein synthesizing systems the translation products are shown to include both SM1 and SM2 myosin heavy chains. These comigrate exactly on 5% SDS polyacrylamide gels with authentic counterparts from ALD muscle. Finally, when peptide maps of SM1 and SM2 myosin heavy chains synthesized in the reticulocyte lysate are compared they are again found to be distinctly different and each is identical to a peptide map of respective authentic SM1 and SM2 myosin heavy chains. It is concluded that the myosin heavy chains of SM1 and SM2 myosin isozymes of ALD muscle have different primary structures and that they are encoded by two distinctly different mRNAs.     

Fibrinoligase-catalyzed cross-linking of myosin from platelet and skeletal muscle.

Fibrinoligase (thrombin- and calcium-activated Factor XIII) from human plasma catalyzes the incorporation of dansylcadaverine and [¹⁴C]putrescine into myosin, prepared from either human platelets or rabbit skeletal muscle. At least 9 mol of amine is incorporated per mole of myosin of either type when the enzyme is used under saturating conditions. Both heavy and light chains of the platelet and muscle myosins incorporate dansylcadaverine and [ ¹⁴C]putrescine. However, in quantitative terms, the incorporation into the light chains of either type is much less than into the heavy chains. Profound fluorescent changes occurred when dansylcadaverine was bound to myosin. Highly cross-linked platelet and muscle myosin polymers form in the absence of added amines, indicating the presence of both acceptor and donor sites. ATPase activity was not altered by cross-linking of 50–60% of myosin. The nature of the cross-link in myosin was found to be a γ-glutamyl-?-lysine bond, with an average of 19 mol of dipeptide per mole of platelet myosin.     

Analysis of myosin light and heavy chain types in single human skeletal muscle fibers

In this study, myosin types in human skeletal muscle fibers were investigated with electrophoretic techniques. Single fibers were dissected out of lyophilized surgical biopsies and typed by staining for myofibrillar ATPase after preincubation in acid or alkaline buffers. After 14C-labelling of the fiber proteins in vitro by reductive methylation, the myosin light chain pattern was analysed on two-dimensional gels and the myosin heavy chains were investigated by one-dimensional peptide mapping. Surprisingly, human type I fibers, which contained only the slow heavy chain, were found to contain variable amounts of fast myosin light chains in addition to the two slow light chains LC1s and LC2s. The majority of the type I fibers in normal human muscle showed the pattern LC1s, LC2s and LC1f. Further evidence for the existence in human muscle of a hybrid myosin composed of a slow heavy chain with fast and slow light chains comes from the analysis of purified human myosin in the native state by pyrophosphate gel electrophoresis. With this method, a single band corresponding to slow myosin was obtained; this slow myosin had the light chain composition LC1s, LC2s and LC1f. Type IIA and IIB fibers, on the other hand, revealed identical light chain patterns consisting of only the fast light chains LC1f, LC2f and LC3f but were found to have different myosin havy chains. On the basis of the results presented, we suggest that the histochemical ATPase normally used for fibre typing is determined by the myosin heavy chain type (and not by the light chains). Thus, in normal human muscle a number of 'hybrid' myosins were found to occur, namely two extreme forms of fast myosins which have the same light chains but different heavy chains (IIA and IIB) and a continuum of slow forms consisting of the same heavy chain and slow light chains with a variable fast light chain composition. This is consistent with the different physiological roles these fibers are thought to have in muscle contraction.     

The light chains of scallop myosin as regulatory subunits

In molluscan muscles contraction is regulated by the interaction of calcium with myosin. The calcium dependence of the aotin-activated ATPase activity of scallop myosin requires the presence of a specific light chain. This light chain is released from myosin by EDTA treatment (EDTA-light chains) and its removal desensitizes the myosin, i.e. abolishes the calcium requirement for the actin-activated ATPase activity, and reduces the amount of calcium the myosin binds; the isolated light chain, however, does not bind calcium and has no ATPase activity. Calcium regulation and calcium binding is restored when the EDTA-light chain is recombined with desensitized myosin preparations. Dissociation of the EDTA-light chain from myosin depends on the concentration of divalent cations; half dissociation is reached at about 10^?5 M-magnesium or 10^?7 M-calcium concentrations. The EDTA-light chain and the residual myosin are fairly stable and the components may be kept separated for a day or so before recombination.Additional light chains containing half cystine residues (SH-light chains) are detached from desensitized myosin by sodium dodecyl sulfate. The EDTA-light chains and the SH-light chains have a similar chain weight of about 18,000 daltons; however, they differ in several amino acid residues and the EDTA-light chains contain no half cystine. The SH-light chains and EDTA-light chains have different tryptic fingerprints. Both light chains can be prepared from washed myofibrils.Densitometry of dodecyl sulfate gel electrophoresis bands and Sephadex chromatography in sodium dodecyl sulfate indicate that there are three moles of light chains in a mole of purified myosin, but only two in myosin treated with EDTA. The ratio of the SH-light chains to EDTA-light chains was found to be two to one in experiments where the total light-chain complements of myosin or myofibril preparations were carboxymethylated. A similar ratio was obtained from the densitometry of urea-acrylamide gel electrophoresis bands. We conclude that a myosin molecule contains two moles of SH-light chain and one mole of EDTA-light chain, and that the removal of a single EDTA-light chain completely desensitizes scallop myosin.Heavy meromyosin and S-1 subfragment can be prepared from scallop myosin. Both of these preparations bind calcium and contain light chains in significant amounts. The heavy meromyosin of scallop is extensively degraded; the S-1 preparation, however, is remarkably intact. Significantly, heavy meromyosin has a calcium-dependent actin-activated ATPase while the S-1 does not require calcium and shows high ATPase activity in its absence. These results suggest that regulation involves a co-operativity between the two globular ends of the myosin.Desensitized scallop myosin and scallop S-1 preparations can be made calcium sensitive when mixed with rabbit actin containing the rabbit regulatory proteins. This result makes it unlikely that specific light chains of myosin are involved in the regulation of the vertebrate system.The fundamental similarity in the contractile regulation of molluscs and vertebrates is that interaction between actin and myosin in both systems requires a critical level of calcium. We propose that the difference in regulation of these systems is that the interaction between myosin and actin is prevented by blocking sites on actin in the case of vertebrate muscles, whereas in the case of molluscan muscles it is the sites on myosin which are blocked in the absence of calcium.     

Isolation of Messenger Ribonucleic Acid for Myosin Heavy Chain

A chicken embryonic polysome fraction that contains 50–60 monoribosomes and synthesizes the heavy chains of myosin is separated from other polysomes of smaller sizes by centrifugation through two cycles of discontinuous and continuous sucrose gradients. The unique properties of the polyadenylic acid segment present at the 3′-end of eukaryotic messenger RNA (mRNA) were used to purify the mRNA for myosin heavy chain from the phenol-extracted total RNA obtained from this polysome fraction. The total RNA was filtered thro ugh millipore filters resulting in partition of the riboscmal RNA (rRNA) and mRNA species. This millipore-bound RNA fraction, which consists of the mRNA and some ribosomal RNAs, was eluted from the filters with sodium dodecyl sulfate (SDS). Subsequent chromatography of this fraction on a cellulose column gave two well-separated peaks: an unadsorbed peak of ribosomal RNAs which was eluted with buffers of high ionic strength and an adsorbed peak of mRNA which was eluted only with a buffer of low ionic strength. Polyacrylamide gel electrophoresis of the mRNA peak fraction showed a single band with no detectable amounts of other RNAs, the mRNA migrating slower than 28S rRNA. The product of in vitro translation of the purified mRNA using a homologous cell-free system was identified as the myosin heavy chain by the following criteria: coprecipitation with carrier myosin at low ionic strength; elution properties on DEAE-cellulose column; and comigration with the heavy chain in polyacrylamide gel electrophoresis. In order to demonstrate the fidelity of translation of the mRNA, ¹⁴C-labeled products of the in vitro translation were copurified with unlabeled myosin heavy chains added as a carrier. The mixture of polypeptides was then cleaved with CNBr and the resulting peptides were separated by molecular sieving. The correlation between the radioactivity and the UV absorbance in the separated peptides indicates that total synthesis of the myosin heavy chain was achieved.     

Stretch-induced growth of skeletal myotubes correlates with activation of the sodium pump

Skeletal myotubes responded to passive stretch by increased amino acid uptake (as measured with [³H]α-aminoisobutyric acid), increased incorporation of amino acids into total cellular protein and myosin heavy chains, and increased accumulation of total cellular protein and myosin heavy chains. These alterations were preceded by an increase in the uptake of ouabain-sensitive rubidium-86 (⁸⁶Rb⁺), a potassium tracer used to measure membrane sodium pump activity (Na⁺K⁺ATPase). This stretch-induced stimulation of ⁸⁶Rb⁺ uptake resulted from a 60-70% increase in the V_max of the Na pump with little change in the K_m. [³H] ouabain binding studies showed no stretch-induced change in the number of membrane Na pumps, indicating that stretch activates the Na pumps that are already present on the cell surface. Since the stretch-induced increases in amino acid transport and amino acid incorporation into proteins were inhibited by ouabain, Na pump activation may be involved in stretch-induced cell growth of skeletal muscle cells by hypertrophy.     

No classical type IIB fibres in dog skeletal muscle

Summary To analyse the fibre type composition of adult dog skeletal muscle, enzyme histochemistry, immunohistochemistry for type I, IIA and IIB myosins, and peptide mapping of myosin heavy chains isolated from typed single fibres were combined. Subdivision of type II fibres into two main classes according to the activity of the m-ATPase after acidic and alkaline preincubation proved to be rather difficult and was only consistently achieved after a very careful adjustment of the systems used. One of these sub-classes of type II fibres stained more strongly for m-ATPase activity after acidic and alkaline preincubation, was oxidative-glycolytic and showed a strong reaction with an anti-type IIA myosin. The other one, however, although unreactive with anti-IIA myosin, was also oxidative-glycolytic, and only showed a faint reaction with an anti-type IIB myosin. Peptide mapping of the myosin heavy chains of typed single fibres revealed two populations of heavy chains among the type II fibre group. Thus, in dog muscle, we are confronted with the presence of two main classes of type II fibres, both oxidative-glycolytic, but differing in the structure of their myosin heavy chains. In contrast to some reports in the literature, no classical type IIB fibres could be detected.     

Evidence for new forms of cardiac myosin heavy chains in mechanical heart overloading and in ageing

Heavy chains of myosin rods and subfragment 1 were isolated from normal hearts and from mechanically overloaded hearts of young and older rats. These myosin heavy-chain fragments were cleaved by cyanogen bromide or partially proteolysed by pronase and by chymotrypsin after denaturation with sodium dodecyl sulfate. The peptides, analyzed by electrophoresis on a one-dimensional polyacrylamide slab gel, varied depending on the origin of the cardiac myosin heavy chains. Some bands present in the peptide patterns of the normal heart of young rats were missing from the pattern of greatly hypertrophied hearts and vice versa. We conclude that mechanical overloading of the heart stimulates the synthesis of cardiac myosin 'isozyme' with a heavy-chain primary structure which is different from that observed in the normal heart of young rat. The patterns from myosin heavy-chain peptides from the hearts of older rats were different from those for peptides from young rat hearts; these results also indicate the presence of a new myosin heavy chain specific to ageing. No difference was detected between the peptide patterns of heavy chains isolated from hypertrophied hearts of young and older rats, and those isolated from normal hearts of older rats.     

White muscle differentiation in the eel (Anguilla anguilla L.): changes in the myosin isoforms pattern and ATPase profile during post-metamorphic development

Myosin isoforms and their light and heavy chains subunits were studied in the white lateral muscle of the eel during the post metamorphic development, in relation with the myosin ATPase profile. At elver stage VI A1 the myosin isoforms pattern was characterized by at least two isoforms, FM3 and FM2. The fast isomyosin type 1 (FM1) appeared during subsequent development. It increased progressively in correlation with the increase in the level of the light chain LC3f. FM1 became predominant at stage VI A4. At the elver stage VI A1, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed at least two heavy chains, namely type II-1 and II-2. The type II-1 heavy chain disappeared in the yellow eel white muscle, and V8-protease peptide map showed the appearance of a minor heavy chain type II-3 as early as stage VI B. Comparison of myosin heavy chains and myosin isoforms patterns showed the comigration of different myosin isoforms during white muscle development. The myosin ATPase profile was characterized by a uniform pattern as far as stage VI A4. A mosaic aspect in white muscle was observed as early as stage VI B, showing the appearance of small acid labile fibers. This observation suggests that the type II-3 heavy chain is specific to the small fibers.     

A comparison of the alkali light chain subunits of vertebrate skeletal muscle myosin in free and heavy chain associated states

The conformations of the alkali light-chain subunits A1 and A2 of vertebrate fast-twitch muscle myosin have been compared for these chains both in their free state and their heavy-chain-associated states by examining the fluorescence parameters of the extrinsic probe 2-(4′-maleimidylanilino)naphthalene-6-sulfonic acid attached covalently to the two light chains. The effect of temperature, salt concentration, and ligands such as Mg²⁺ ions, MgADP, and MgATP has also been investigated. In spite of the extensive sequence homology between the two light chains the data indicate that in their free states the fluorophore in the A2 chain resides in a considerably higher hydrophobic environment. It was also found that the presence of the bulky fluorophore on these light chains does not adversely affect their ability to hybridize with Subfragment 1 heavy chains to form ATPase active hybrids. This association to the heavy chains is accompanied by significant changes in the quantum yields of the 2-(4′-maleimidylanilino)naphthalene-6-sulfonic acid label indicating that conformational changes do occur during this transition. Mg²⁺ ions were found to cause either an enhancement or a decrease in fluorescence intensity depending on whether the alkali light chains were free or combined to the heavy chains, respectively. Fluorescence perturbation by nucleotide was only observed for the heavy-chain-associated state.     

Myofibrillar proteins in skeletal muscles of parr,smolt and adult atlantic salmon (Salmo salarl.). Comparison with another salmonid,the arctic charr Salvelinus alpinus (l.)

The heavy and light subunits of myosin from white and red muscles of Atlantic salmon parr, smolt and adult individuals were analyzed by SDS-PAGE and two-dimensional electrophoresis. Tropomyosin was identified by comigration with rat tropomyosins in two-dimensional gels in the presence and absence of urea. These myofibrillar proteins were compared to those of Arctic charr.

• 1.1. The myosin heavy chain from Atlantic salmon red muscles was associated with two types of light chain, 1S and 2S, that comigrated with the light chains 1S and 2S of Arctic charr.
• 2.2. As in the Arctic charr, four myosin light chain spots were detected in white muscles: two fast myosin light chains type 1, one of which comigrated with its analogous in the Arctic charr; one fast myosin light chain type 2, differing slightly in isoelectric point from that of Arctic charr; and one fast myosin light chain type 3 with higher electrophoretic mobility than that of Arctic charr.
• 3.3. Three tropomyosin spots were detected. White muscles contained only one type of β-tropomyosin and red muscles two types of α-tropomyosin. These three tropomyosin spots comigrated with those of Arctic charr.
• 4.4. Two myosin heavy chain bands were observed in red muscles of salmon parrs but only one in the rest of the red muscles analyzed.
• 5.5. Only one myosin heavy chain band was detected in white muscles by SDS-glycerol-polyacrylamide gel electrophoresis. Alfa-chymotryptic peptide mapping of these white myosin heavy chain bands revealed differences attributed to the presence of a new type of myosin heavy chain first detected several months after smoltification.