Myosin Type Research Articles

Thermal scanning rheology of myofibrillar proteins from muscles of defined fibre type

Emulsion meat products are made with little regard to rheological properties of different muscles. Here the rheology of gels made from three classes of muscle defined by myosin type (fast twitch, slow twitch and heart) are compared. Myofibrillar fractions were prepared from representative bovine muscles— cutaneus trunci, masseter and heart—by a procedure that removed connective tissue, fat and sarcoplasmic proteins. Complicating effects of nongelling agents were thus minimized. Fractions were mixed with NaCl and pyrophosphate at concentrations typical of those used in processed meats. Rigidity and elasticity of the gelling mixtures were monitored during heating from 10 to 84°C over a range of pH values. Several indices of gelation showed that masseter and heart (slow muscle group) gelations were similar to each other but distinct from cutaneus trunci (fast) gelation. Gelation temperature was 10°C lower for fast than slow, perhaps explainable by one of two hypotheses: differential salt extraction or lower thermal stability of fast myosin rod. Cutaneus trunci gels were also more rigid at all pH values. Slow group gels suffered from proteolysis, which was combated with cathepsin inhibitors. For both fast and slow groups, rigidity increased as pH decreased, analogous to results reported for purified myosin in dilute solution. Texture also changed. Below pH 5·7 slow group gels were brittle and granular, with low water-holding capacity. Above that pH, gels were elastic and smooth. The equivalent point for cutaneus trunci gels was pH 5·5. Around pH 7, pyrophosphate had a fluidizing effect at pregelation temperatures, but elasticity data indicated that this ion plays no part in final gelation.

Cardiac Adaptation and Its Limitation in an Experimental Model of Congestive Heart Failure.

To elucidate mechanisms of adaptation and maladaptation in heart failure, abnormalities of left ventricular function and their relationships to myocardial contractile protein were studied in the Syrian hamster Bio 14.6. Left ventricular and heart weights were both increased in 20-week-old cardiomyopathic hamsters, indicating cardiac hypertrophy as a compensatory mechanism to the disease process of cardiomyopathy. However further increase in the left ventricular weight was not observed in older (40-week-old) cardiomyopathic hamsters. On the other hand left ventricular volume and volume/mass ratio were increased progressively. Correspondingly, V3 type myosin was increased and myosin sliding velocity was decreased. Left ventricular function of cardiomyopathic hamsters evaluated using an isovolumically beating perfused heart preparation was depressed, and this functional impairment was also progressive. Chronic administration of metoprolol, a beta-blocking agent, induced further increase in left ventricular volume and mass without changing left ventricular function and myosin isozyme pattern. Thus in cardiomyopathic hamsters, left ventricular function progressively deteriorates in spite of a variety of adaptive mechanisms, and remodeling occurs.

Motor units of the fourth deep lumbrical muscle of the adult rat: isometric contractions and fibre type compositions.

1. Isometric twitch and tetanic tensions were recorded from whole muscles and single motor units in fourth deep lumbrical muscles isolated from young adult (60 days) rats. Muscles were superfused with oxygenated Ringer solution at 25 degrees C except where stated otherwise. 2. It was confirmed that the muscle is supplied most commonly by eleven motor axons, nine via the lateral plantar nerve (LPN), and two via the sural nerve (SN). Motor units whose axons were isolated from either LPN or SN were studied. There was no difference in mean motor unit size. 3. In their unfused tetani most units showed 'sag' and some 'no sag', with no segregation between LPN and SN. 'No sag' units were always small (unit tetanic tension less than 8% whole-muscle tetanic tension), tended to be relatively slowly contracting and relaxing during an isometric twitch, and tended to have relatively low twitch:tetanus ratios. Units showing sag ranged from large to small. 4. In some motor units muscle fibres were depleted of their glycogen by repetitive stimulation at 30 degrees C in glucose-free Ringer solution, and the muscle and its unstimulated control frozen and sectioned. Neighbouring sections were stained for glycogen and for binding of two myosin-specific antibodies, one specific for slow myosin and the other for type IIA myosin. Myosin ATPase and succinic dehydrogenase histochemistry were also carried out in some muscles. 5. Serial reconstructions showed that all or virtually all extrafusal fibres in the muscle were present in a midbelly section, and that the myosin type of individual fibres did not change significantly along their length. Spindle profiles were seen frequently and in two muscles eight and twelve spindles were identified. 6. Of twenty-six motor units examined twenty contained almost exclusively muscle fibres of the recently described type IIX. All these units showed sag in their isometric tetani. 7. Six units each contained 50% or more of slow myosin-containing fibres (IIC and a few type I). The remaining fibres in these units were IIA. All these units were therefore of mixed fibre composition, and are discussed as IIC/IIA units. In whole muscles slow-myosin-containing fibres were generally distributed evenly (non-randomly) throughout the muscle cross-section. 8. Whole muscles contained on average 970 fibres (S.D. +/- 70) of which 82 (+/- 9) were slow-myosin-containing. A few muscles from older rats (3-24 months) contained very few such fibres.(ABSTRACT TRUNCATED AT 400 WORDS)

Uniform myosin isoforms in the flight muscles of little brown bats, Myotis lucifugus

The present study used muscle histochemistry and polyacrylamide gel electrophoresis of native myosin and myosin heavy chains to establish a correlation, if any, between chiropteran histochemical fiber types and myosin isoform composition. Histochemical analysis of the primary flight muscle, the pectoralis profundus, documented the presence of a single histochemical fiber type, here termed Type II. Electrophoresis of native myosin isolated from pectoralis muscle yielded a single isoform that comigrated with the FM-3 isoform of rat diaphragm. Heavy chain analysis of the Myotis pectoralis demonstrated a single heavy chain with comparable electrophoretic mobility to rat IIa myosin heavy chain. These data demonstrate unique histochemical and biochemical homogeneity in the myosin composition of the pectoralis muscle of Myotis lucifugus. Thus this muscle is extremely specialized for flight at histochemical, morphologic, and molecular levels. These data contrast with the mixed myosin and histochemical fiber types found in other mammals, as well as in other muscles of Myotis lucifugus.

Competitive control of myosin expression: hypertrophy vs. hyperthyroidism

The competition between two opposing influences on the phenotypic expression of skeletal muscle myosin were studied to determine which was the dominant regulator. Experimental hyperthyroidism, which induces fast myosin expression, was produced by subcutaneous implantation of a 40-day constant-time-release triiodothyronine pellet. Compensatory hypertrophy, which induces slow myosin expression, was produced by surgical removal of a synergistic hindlimb muscle. Hyperthyroidism increased the percentage of type II fibers and the fast myosin isoforms in both the plantaris and soleus muscles. Hypertrophy significantly increased the percentage of type I fibers and the slow myosin type in the plantaris and soleus muscles. However, with the simultaneous introduction of hyperthyroidism and hypertrophy, only the hyperthyroid effects were observed. Hyperthyroidism and not physiological demand was found to be the dominant regulator of skeletal muscle myosin expression.

Regulation of cytoplasmic and smooth muscle myosin

Phosphorylation is commonly the regulator of smooth muscle and cytoplasmic myosins, but the site and mechanism of this regulation varies widely between different species and between myosin types.

Myocardial biochemical and hemodynamic adaptations to chronic tachycardia

The purpose was to determine the biochemical and hemodynamic adaptations of the myocardium to chronic tachycardia. Cardiac pacemakers were implanted in Yorkshire pigs and set at a rate of 180 beats/min for a period of 35-42 days. Animals were then anesthetized with pentobarbital sodium. Myocardial blood flow and hemodynamics were determined at three different heart rates (i.e., 120, 180, and 220 beats/min). Tissue samples were then taken for microsphere and biochemical analyses. Chronically paced hearts maintained better cardiac function and had consistently higher left ventricular blood flow with a higher endocardial-to-epicardial ratio. The activities of citrate synthase and 3-hydroxyacyl-CoA dehydrogenase were 23 and 45% greater in the paced hearts, respectively. The sarcoplasmic reticulum adenosinetriphosphatase activity was 55% greater in the paced hearts, whereas the myosin adenosinetriphosphatase was the same as in the control hearts. Polyacrylamide gels of the ventricular myosin isoforms showed only the V3 type to be present in both the control and paced hearts. These findings show that the heart of a large mammal adapts to chronic tachycardia (i.e., 180 beats/min) by elevating the aerobic and calcium-sequestering capacities without altering its myosin type.

Comparison of myosin isoenzymes present in skeletal and cardiac muscles of the Arctic charr Salvelinus alpinus (L.)

The expression of myosin isoforms and their subunit composition in the white skeletal body musculature of Arctic charr (Salvelinus alpinus) of different ages (from 77-day embryos until about 5 years old) was studied at the protein level by means of electrophoretic techniques. Myosin from the white muscle displayed three types of light chain during all the developmental stages examined: two myosin light chains type 1 (LC1F) differing in both apparent molecular mass and pI, one myosin light chain type 2 (LC2F) and one myosin light chain type 3 (LC3F). The fastest-migrating form of LC1F seemed to be predominant during the embryonic and eleutheroembryonic periods. The slowest-migrating form of LC1F was predominant in the 5-year-old fish. Between 1 year and 4 years, both types of LC1F were present in similar amounts. Cardiac as well as red muscle myosin from 3-year-old fish had two types of light chain. The myosin light chains from atria and ventriculi were indistinguishable by two-dimensional electrophoresis, but were different from the myosin light chains from red muscle. Neither the light chains from cardiac nor red muscle were coexpressed with the myosin light chains of white muscle at any of the developmental stages examined. Two myosin heavy chain bands were resolved by SDS/glycerol/polyacrylamide gel electrophoresis of the extract from embryos. One of the bands was present in minor amounts. The other, and most abundant, band comigrated with the only band found in the extracts of white muscle myosin from older fish. One-dimensional Staphylococcus aureus V8 protease peptide mapping of these bands revealed some differences during development of the white muscle tentatively interpreted as follows. The myosin heavy chain band present in minor amounts in the embryos may represent an early embryonic form that is replaced by a late embryonic or foetal form in the eleutheroembryos. The foetal myosin heavy chain appears to be present until the resorption of the yolk sack and beginning of the free-swimming stage. A new form of myosin heavy chain, termed neonatal and probably expressed around hatching, is present until about 1 year of age.(ABSTRACT TRUNCATED AT 400 WORDS)

Myosin I: A new insight into the mechanism and cellular significance of actin-based motility

From our work on brush border myosin I structure, activity, regulation, and function, we can begin to understand the significance of the diversification of myosin proteins. While myosin I and II proteins retain conserved elements of structure that may dictate their similar mechanisms of motility and actin-activated MgATPase activity, their unique structures may provide the basis for the distinct localization and regulation of the two myosin types. How does the tropomyosin-inhibited actin-binding site of myosin I differ from that of the tropomyosin-activated myosin II actin-binding site? What elements of the sites of interaction of the 110K-protein and calmodulin contribute to the conserved, light-chain dependent coupling of MgATPase activity to translocation and which confer the novel calcium regulation of dissociation in vitro? It seems that the evolutionary demand for diversification of cellular motility functions has been met, at least in the actin-based system, by the evolution of isoforms tailored in structure, activity, regulation, and localization to serve complementary needs.

Regulation of skeletal muscle fiber size, shape and function

There is convincing evidence that the cross-sectional area, the type of myosin expressed, the potential for oxidative phosphorylation and the number of myonuclei of a skeletal muscle fiber are closely interdependent. Each of these variables, as well as the shape of the fiber, has identifiable physiological consequences. Further, it is suggested that the cytoplasmic to myonucleus ratio is a function of the myosin type and the amount and/or rate of protein synthesis and degradation. Although the neuromuscular activity (electromyographic activity) as well as the associated mechanical and metabolic events have significant regulatory influences on protein metabolism, there are other important regulatory factors independent of these activity-related events. Both the activity and non-activity related regulatory mechanisms probably occur via a cascade of cellular events. The specific combinations of cellular responses that occur may define the nature of the modulatory effects on specific proteins. In spite of the complexity of the regulatory mechanisms of protein modulation and how these responses are structurally integrated into or removed from functional fibers, it is suggested that controlled studies of human neuromuscular function can be more accurately defined and interpreted when fiber and muscle size and shape are considered.

A new Acanthamoeba myosin heavy chain. Cloning of the gene and immunological identification of the polypeptide

An Acanthamoeba myosin heavy chain has been identified whose tail domain amino acid sequence distinguishes it from Acanthamoeba myosins IB, IC, and II. The gene for this novel myosin heavy chain spans approximately 6.8 kilobases, is split by 17 introns, and encodes a 177-kDa polypeptide. While the amino-terminal approximately 90 kDa of this polypeptide is highly similar to the globular head sequences of myosins I and II, its approximately 87-kDa tail domain shows essentially no similarity to the tail sequences of either type of myosin. The only exception to this is the carboxyl-terminal approximately 50-amino acid region of the polypeptide, which is homologous to the carboxyl termini of the myosins I. Interestingly, this approximately 50-residue segment has been shown to exist in a diverse family of cytoskeleton-associated proteins that include nonreceptor tyrosine kinases, phospholipase C gamma, and fodrin (Rodaway, A. R. F., Sternberg, M. J. E., and Bentley, D. L. (1989) Nature 342, 624). Sequence analysis indicates that the tail domain of this new myosin is incapable of forming a myosin II-like coiled-coil structure, implying that the protein is single-headed and nonfilamentous. For this reason we have tentatively classified it as a high molecular weight form of myosin I (HMWMI). To determine if HMWMI exists in cells, antiserum was raised against a bacterially expressed fusion peptide made using a cDNA clone encoding most of the unique HMWMI tail domain. This antiserum does not recognize Acanthamoeba myosins IB, IC, or II but does recognize a single polypeptide in whole cell extracts with the mobility predicted for the HMWMI heavy chain. This protein is precipitated from crude extracts using F-actin and released from the pellet by ATP, supporting its classification as a member of the myosin family of proteins.

Comparative structural and enzymatic properties of skeletal muscle myosin from neonatal and adult rabbits

Several structural and enzymatic properties of myosin from skeletal muscles of neonatal and adult rabbits were compared. Electrophoretic analyses and proteolysis experiments indicated that differences between the two myosin types could be attributed to their heavy subunits. Circular dichroism measurements of subfragment-1 species, and trypsin-digested derivatives showed that the neonatal protein contained less alpha-helices than the adult form. The Mg2(+)-ATPase activity of neonatal myosin was lower than that of adult myosin, especially in the presence of actin. In comparison with adult subfragment-1, it was found that the binding of ATP analogues such as adenosine 5'-[beta, gamma-imino]triphosphate and PPi, or that of ATP (as deduced from the apparent KmATP) to neonatal subfragment-1 in the presence of actin was enhanced, while that of ADP was decreased. On the other hand, the association of actin with the ADP - neonatal-subfragment-1 complex was weaker. These features must be expressed in the cyclical actin-myosin association/dissociation steps occurring in ATP hydrolysis, and more particularly in the reassociation of actin with the ATP-hydrolysis-products - myosin complex.

Myosin structure and thyroidian control of myosin synthesis in urodelan amphibian skeletal muscle.

Electrophoretic analysis in non-dissociating conditions reveals three types of myosin in adult urodelan amphibian skeletal muscles: 3 isoforms of fast myosin (FM), one isoform of intermediate myosin (IM) and one or two isoforms of slow myosin (SM). Each type is characterized by a specific heavy chain HCf (FM), HCi (IM) and HCs (SM), respectively. In all urodelan species, as in mammals, fast isomyosins associate HCf and the three fast light chains LC1f, LC2f, and LC3f. In most urodelan species the intermediate myosin contains LC1f and LC2f and can be considered as an homodimer of the alkali LC1f. However, in Euproctus asper, IM is characterized by the association of both slow and fast LC with HCi. Slow myosin is a hybrid molecule associating HCs with slow and fast LC. During metamorphosis, a myosin isoenzymic transition occurs consisting in the replacement of three larval myosins (LM) characterized by a specific heavy chain (HCI), by the adult isomyosins with lower electrophoretic mobilities. At the same time there is a change in the ATPase myofibrillar pattern, with the larval fiber types being replaced by adult fibers of types I, IIA and IIB. In the neotenic and perennibranchiate species, which do not undergo spontaneous metamorphosis, sexually mature larval animals present a change in the myosin isoenzymic profile, but no complete transition. The coexistence of larval and adult isomyosins and the persistence of transitional fibers of type IIC in the skeletal muscle are demonstrated. Experimental hypo- and hyperthyroidism indicate that thyroid hormone stimulates the regression of the larval isomyosins, possibly through indirect pathways. In contrast, the appearance and the persistence of the adult isomyosins seem to be independent of thyroid hormone. Thus, the control of the isoenzymic transition in the skeletal muscle of urodelan amphibians appears to imply indirect mechanisms, operating differently on each of the two phases of the complete transition.

Regulation of Drosophila myosin ATPase activity by phosphorylation of myosin light chains—I. Wild-type fly

1. 1. Two types of myosins with phosphorylated and dephosphorylated myosin light chains were prepared from Drosophila flies. The former had ATPase ( Ca 2+- and Mg 2+-activated ) activities twice those of the latter. 2. 2. The myosin phosphorylated with crude myosin light chain kinase from flies showed ATPase ( Ca 2+- and Mg 2+-activated ) activaties twice those of the dephosphorylated myosin. 3. 3. It is suggested that phosphorylation of myosin light chains several hours after emergence stimulates myosin ATPase activity so as to facilitate the flight function of the fruitfly.

Electrophoretic study of myosin isoforms in white muscles of some teleost fishes

1. 1. White skeletal muscle myosin of four marine teleost fish species (cod, blue whiting, Norway haddock, and spotted wolf-fish) was analyzed by native, SDS-PAGE, and 2-dimensional electrophoresis. 2. 2. Four types of native myosin were present in cod, blu whiting and Norway haddock. The second fastest migrating form was predominant. 3. 3. Myosin from spotted wolf-fish also resolved into four forms. The fastest migrating form was hardly noticeable. The other three were present in apparently similar amounts. 4. 4. In the myosin from each species there were three types of light chains. The pattern of light chains was species specific. 5. 5. Apparently, there was only one type of heavy chain in myosin from cod, Norway haddock and spotted wolf-fish. One preparation of cod showed an extra band of higher electroporetic mobility than the main band. In blue whiting we found two bands present in approximately equal amounts.

Phosphorylated and dephosphorylated myosin light chains of Drosophila fly and larva

1. 1. The present study confirmed that light chains of Drosophila adult fibrillar (flight) muscle myosin consist of Lf1, Lf2, Lf2′ and Lf3, and tubular muscle myosin light chains contain Lt1, Lt2 Lt2′ and lt3, as revealed by two-dimensional (isoelectric focusing and SDS-gel electrophoresis) gel electrophoresis. 2. 2. larva myosin light chains were of all the tubular type. However, it was found that Ltl and Lt2′ are produced by phosphorylation of Lt2, and Lf1 is produced by phosphorylation of Lf2′. 3. 3. Injection of radioactive phosphate into Drosophila fly resulted in phosphorylations of Lf1 and Lt1. When larva or late pupa myosin was incubated with myosin light chain kinase from chicken gizzard or adult flies, phosphorylation of Lt1, Lf2′ and Lt2′ occurred. Drosophila myosin light chain kinase phosphorylated Lf1 in addition to Lt1 and L2′ ( Lf2′ + Lt2′) of adult myosin. 4. 4. Dephosphorylation of adult myosin by potato acid and calf intestine alkaline phosphatases led to the shift of Lf1 (34,000), Lt1 (31,000) and L2′ ( Lf2′ + Lt2′) (30,000 to L2 ( Lf2 + Lt2) positions (30,000). 5. 5. Peptide mapping analyses revealed that larva Lt1, Lt2′, Lt2 and adult Lt1 were all the same; therefore, it is thought that a single species of Lt2 specific to the tubular type of myosin and its phosphorylated isoforms (Lt1, Lt2′) exist. 6. 6. The peptide map of Lf1 was slightly different from that of Lt1, but very similar to that of L2′ in adult myosin. L2 and L2′ of adult myosin showed very similar peptide maps, but there were several different peptide fragments. 7. 7. There were a number of peptides common to L1 (Lf1, Lt1) and L2 (Lf2′, Lt2′, Lf2 and Lt2) but not to L3 (Lf3, Lt3). 8. 8. It is concluded that Lf2′ ( = Lf1), Lf2 (fibrillar muscle) and Lt2 (= Lt1 = Lt2′) (tubular muscle) are the regulatory light chains of Drosophila myosin and that Lf3 (fibrillar) and Lt3 (tubular) are the essential light chains in view of the present findings as well as of the reported results of the gene analyses.

Metabolic and morphologic properties of single muscle fibers in the rat after spaceflight, Cosmos 1887

The adaptation of a slow (soleus, Sol) and a fast (medial gastrocnemius, MG) skeletal muscle to spaceflight was studied in five young male rats. The flight period was 12.5 days and the rats were killed approximately 48 h after returning to 1 g. Five other rats that were housed in cages similar to those used by the flight rats were maintained at 1 g for the same period of time to serve as ground-based controls. Fibers were classified as dark or light staining for myosin adenosine triphosphatase (ATPase). On the average, the fibers in the Sol of the flight rats atrophied twice as much as those in the MG. Further, the fibers located in the deep (close to the bone and having the highest percentage of light ATPase and high oxidative fibers in the muscle cross section) region of the MG atrophied more than the fibers located in the superficial (away from the bone and having the lowest percentage of light ATPase and high oxidative fibers in the muscle cross-section) region of the muscle. Based on quantitative histochemical assays of single muscle fibers, succinate dehydrogenase (SDH) activity per unit volume was unchanged in fibers of the Sol and MG. However, in the Sol, but not the MG, the total amount of SDH activity in a 10-microns-thick section of a fiber decreased significantly in response to spaceflight. Based on population distributions, it appears that the alpha-glycerophosphate dehydrogenase (GPD) activities were elevated in the dark ATPase fibers in the Sol, whereas the light fibers in the Sol and both fiber types in the MG did not appear to change. The ratio of GPD to SDH activities increased in the dark (but not light) fibers of the Sol and was unaffected in the MG. Immunohistochemical analyses indicate that approximately 40% of the fibers in the Sol of flight rats expressed a fast myosin heavy chain compared with 22% in control rats. Further, 31% of the fibers in the Sol of flight rats expressed both fast and slow myosin heavy chains compared with 8% in control rats. Immunohistochemical changes in the MG were minimal. These data suggest that the magnitude and direction of enzymatic activity and cell volume changes are dependent on the muscle, the region of the muscle, and the type of myosin expressed in the fibers. Further, the ability of fibers to maintain normal or even elevated activities per unit volume of some metabolic enzymes is remarkable considering the marked and rapid decrease in fiber volume.

The effect of load on the phenotype of the developing rat soleus muscle

In newborn Wistar rats the load on the soleus muscle was reduced by removing the tibialis anterior (TA) and extensor digitorum longus (EDL) muscles. Eighteen days later the soleus muscles were removed from both the operated and control legs and examined physiologically and histologically. The time course of twitch contraction of the soleus on the operated side was not significantly different from that of control muscles, but the muscles developed less tension. The decreased tension was consistent with a smaller number of muscle fibres. Histochemical and immunocytochemical examination showed that in the operated muscle, fewer fibres reacted with an antibody against slow myosin, while the number of fibres that reacted for alkali-preincubated ATPase, indicative of neonatal or adult fast myosin, was increased. Some fibres expressed both types of myosin. These findings suggest that a reduced load delays the phenotypic expression of slow myosin isoform in the developing soleus muscle.

Administration of a creatine analogue induces isomyosin transitions in muscle.

A creatine analogue, beta-guanidinopropionic acid (beta-GPA), was administered in the food (2% wt/wt) and the water (0.5% wt/vol) of male CD-1 mice. Uptake of the phosphorylated analogue and depletion of phosphocreatine in hindlimb muscle was monitored by 31P nuclear magnetic resonance and was found to be complete within 7 wk. After this time, the isomyosin composition of soleus, extensor digitorum longus (EDL), and ventricle was analyzed by pyrophosphate gel electrophoresis. The analogue was found to induce significant alterations in the type of myosin expressed in soleus and EDL. Normal soleus contains both intermediate (IM) and slow (SM) myosins, and treatment reduced the relative content of IM by approximately 50%. In EDL, treatment decreased fast isomyosin FM3 by 60% compared with controls. Sodium dodecyl sulfate-gel electrophoresis also showed a decrease of parvalbumin in EDL by approximately 50%. Treatment had no significant effect on the isomyosin composition of heart ventricle. Levels of physical activity and concentrations of serum glucose and thyroxine of treated mice were not significantly different from controls. These results indicate a role for intracellular energetics in mediating adaptive changes in the phenotype of muscle in mature animals.

Immunocytochemical localization of myosin, tropomyosin and actin in vestibular hair cells of human fetuses and cats

Semithin sections (1 μm) of human fetuses and young cat vestibular epithelium embedded in Epon were reacted with primary antibodies raised against myosin, tropomyosin and actin. The results were very similar in the two species. Myosin and tropomyosin were colocalized and strictly limited to an apical superficial corona arranged in a punctiform pattern. The stereocilia and cuticular plate were unreactive to myosin and tropomyosin antibodies. Actin antibodies stained the stereocilia, cuticular plate and a marginal ring surrounding and underlying the cuticular plate. At this level, myosin and tropomyosin were not detected. This result suggests that the circumferential actin ring has a structural role. Colocalization of myosin, tropomyosin and actin in a superficial area around the stereocilia bundle and along the apical surface of the hair cell imply that active processes could exist at the apex of certain sensory vestibular hair cells. The specificity of myosin antibodies and their cross-reactivity with different types of myosin are discussed.