Mode of degradation of myofibrillar proteins by rabbit muscle cathepsin D

The mode of degradation of myofibrillar proteins by the action of highly purified rabbit muscle cathepsin D (EC 3.4.23.5) was studied using SDS-polyacrylamide gel electrophoresis. Cathepsin D optimally degraded myosin heavy chain, α-actinin, tropomyosin, troponin T and troponin I at around pH 3. It did not degrade actin or troponin C. Degradation of myosin heavy chain produced four major fragments of 155 000, 130 000, 110 000 and 90 000 daltons. Troponin T was hydrolyzed to 33 000-, and 20 000- and 11 000-dalton fragments. Troponin I was degraded into fragments of 13 000 and 11 000 daltons. Degradation of α-actinin and tropomyosin was not as rapid as that of mysoin and troponins T and I. Tropomyosin gave a fragment of 30 000 daltons, but α-actinin showed no distinct band of this fragment on gels.     

Localization of Myosin,Actin, α-Actinin,Tropomyosin and Vinculin in Surface-Activated,Spreading Human Platelets

We observed the localization of the contractile proteins myosin, filamentous actin, α-actinin, tropomyosin, and vinculin in surface-activated, spreading human platelets using a single fluorescence staining procedure and conventional fluorescence microscopy. Myosin was distributed in a speckled pattern that extended radially from the granulomere. F-actin demonstrated cable-networks. Tropomyosin and α-actinin occurred in a punctuate distribution, and vinculin was localized at adhesion sites. Although myosin, F-actin, α-actinin, tropomyosin, and vinculin were not studied in resting platelets, our data support the idea that these contractile proteins are reorganized and reassembled in activated platelets during platelet function.     

Accumulation of myosin, actin, tropomyosin, and alpha-actinin in cultured muscles cells.

In an effort to understand the conditions that promote the assembly of myofibrillar proteins in muscle cells, the temporal sequence of accumulation of four myofibrillar proteins, actin, myosin, tropomyosin, and α-actinin, was monitored during the period of de novo assembly of myofibrils in differentiating muscle cells. Isotope dilution experiments indicated that all four proteins were accumulated simultaneously. Therefore, assembly of myofibrils may be occurring in the presence of a full complement of myofibrillar proteins.     

Studies on purified α-actinin : I. Effect of temperature and tropomyosin on the α-actinin/F-actin interaction

α-Actinin, purified by two passages through a DEAE-cellulose column, migrates either as a single band or as a single major band with a fainter trailing band during polyacrylamide gel electrophoresis at pH 8.3. In the presence of sodium dodecyl sulfate, purified α-actinin always migrates as a single electrophoretic zone during polyacrylamide gel electrophoresis. Temperature has large effects on the interaction of α-actinin with F-actin. At 0 °C, α-actinin causes large increases in F-actin viscosity, either in the presence or absence of tropomyosin. Quantitative binding studies show that α-actinin can displace tropomyosin from F-actin at 0 °C and that F-actin will quantitatively bind 45% of its weight of α-actinin either in the presence or absence of tropomyosin. This binding ratio corresponds to one α-actinin molecule to approximately 10 to 11 G-actin subunits and suggests that one molecule of α-actinin binds to each turn of the F-actin helix at 0 °C.     

ALTERATIONS IN DISTRIBUTION OF ACTIN BINDING PROTEINS IN UTERINE STROMAL CELLS DURING DECIDUALIZATION IN THE RAT

Immunohistochemistry was used to investigate the involvement of the actin-associated binding proteins, tropomyosin, α-actinin and gelsolin with the formation of the decidual cell reaction during early pregnancy in the rat. Tropomyosin was present in the uterine myometrium, but absent from the both decidual and non-decidual stromal cells. α-Actinin was absent from non-decidual stromal cells, but present in decidual cells. Gelsolin was present in non-decidual cells close to the uterine stroma as well as in transformed decidual cells. Both gelsolin and α-actinin were concentrated around the periphery of the cell. It is proposed that these actin-binding proteins may be involved with the cellular transformations associated with decidualization.     

Study of reciprocal effects of cardiac myosin and tropomyosin isoforms on actin-myosin interaction with in vitro motility assay

Interaction of myosin with actin in striated muscle is controlled by Ca²⁺ via thin filament associated proteins: troponin and tropomyosin. In cardiac muscle there is a whole pattern of myosin and tropomyosin isoforms. The aim of the current work is to study regulatory effect of tropomyosin on sliding velocity of actin filaments in the in vitro motility assay over cardiac isomyosins. It was found that tropomyosins of different content of α- and β-chains being added to actin filament effects the sliding velocity of filaments in different ways. On the other hand the velocity of filaments with the same tropomyosins depends on both heavy and light chains isoforms of cardiac myosin.     

The functional characteristics conserved in tropomyosins.

1. Tropomyosin, one of the regulatory proteins in muscle contraction, was prepared from chickens, rabbits, frogs, shrimps, and shellfish, and conserved characteristics were studied using an enzymological technique. 2. All tropomyosins tested, irrespective of their sources, were found to have the ability to mediate the inhibitory activity of rabbit troponin toward rabbit Mg2+-activated actomyosin ATPase (Mg2+-ATPase) activity in the absence of Ca2+ ions. 3. The effect of tropomyosin on the Mg2+-ATPase activity in the presence of Ca2+ ions varied, depending on the source, and this variation appeared to reflect the evolutionary course of this protein. 4. Tropomyosin from shellfish adductor muscle had the ability to bind to rabbit skeletal muscle troponin and actin. This ability is also considered to be a basic characteristic of tropomyosin which has been conserved during evolution.     

In vitro labeling of proteins by reductive methylation: Application to proteins involved in supramolecular structures

Actin and tropomyosin, purified from both muscle and brain, and α-actinin, purified from muscle, have been labeled in vitro by reductive methylation to specific activities of greater than 10⁵ dpm/μg protein. Actin so modified bound DNase I and polymerized identically to unmodified actin. Furthermore, the spectral properties of actin did not change after labeling. The interactions of labeled tropomyosin and α-actinin with F-actin were nearly identical to those of the unmodified proteins. These modified proteins comigrated with their unmodified counterparts in both SDS-containing polyacrylamide gels and isoelectric focusing gels. The labeled actin was quantitatively extracted from SDS-containing polyacrylamide gels (yield > 98% of radioactivity applied demonstrating that all of the radioactivity was protein bound. The reductive methylation procedure worked well at pH 8.0–8.5 in either pyrophosphate buffer or Bicine buffer using formaldehyde with [³H]-sodium borohydride as the reducing agent. The procedure could also be performed at pH 7.0 in phosphate buffer using [¹⁴C]-formaldehyde with sodium cyanoborohydride as the reducing agent. Proteins so labeled are ideal for use in quantitative experiments involving protein-protein interactions.     

Study of regulatory effect of tropomyosin on actin-myosin interaction in skeletal muscle by in vitro motility assay

The interaction between myosin and actin in striated muscle tissue is regulated by Ca²⁺ via thin filament regulatory proteins. Skeletal muscle possesses a whole pattern of myosin and tropomyosin isoforms. The regulatory effect of tropomyosin on actin-myosin interaction was investigated by measuring the sliding velocity of both actin and actin-tropomyosin filaments over fast and slow skeletal myosins using the in vitro motility assay. The actin-tropomyosin filaments were reconstructed with tropomyosin isoforms from striated muscle tissue. It was found that tropomyosins with different content of α-, β-, and γ-chains added to actin filaments affect the sliding velocity of filaments in different ways. On the other hand, the sliding velocity of filaments with the same content of α-, β-, and Γ-chains depends on myosin isoforms of striated muscle. The reciprocal effects of myosin and tropomyosin on actin-myosin interaction in striated muscle may play a significant role in maintenance of effective work of striated muscle both during ontogenesis and under pathological conditions.     

Calcium-dependent muscle contraction in obliquely striated Ascaris suum muscle

The regulatory proteins of Ascaris suum striated skeletal muscle were partially purified and characterized. A tropomyosin isoform (Mr 41K) and three troponin subunits identified as troponin T (Mr 37.5K), troponin I (Mr 25.5K) and troponin C (Mr 18.5K) were purified. Three myosin light chains (Mr 25K, 19K, and 17K) were isolated from washed Ascaris actomyosin; the 19K subunit was phosphorylated in vitro. A calcium/calmodulin-dependent myosin light chain kinase activity was identified in the muscle. In contrast to previously reported data suggesting that Ascaris obliquely striated muscle contraction is regulated by a myosin-mediated mechanism, these data indicate that all of the proteins required for actin-mediated, calcium-dependent muscle contraction are present in this tissue.     

Effect of denervation on the isoform transitions of tropomyosin, troponin T, and myosin isozyme in chicken breast muscle

The effect of innervation on the transition of tropomyosin, troponin T, and myosin isozyme during chicken breast muscle development was examined by denervating the muscle at various ages after hatching. The types of proteins were characterized by 2-D electrophoresis for tropomyosin, immunoblotting for troponin T and pyro-phosphate acrylamide gel electrophoresis for myosin isozymes. As judged by the types of these three proteins, when neonatal muscle was denervated, the protein isoform transition from the neonatal to adult state was interrupted, whereas the denervation of mature muscle caused the reappearance of the neonatal forms of proteins. The present results indicate that differentiation from the neonatal state to the adult state and the maintenance of the adult state are controlled by some factors related to nerves.     

Structural basis for the regulation of muscle contraction by troponin and tropomyosin

The molecular switching mechanism governing skeletal and cardiac muscle contraction couples the binding of Ca²⁺ on troponin to the movement of tropomyosin on actin filaments. Despite years of investigation, this mechanism remains unclear because it has not yet been possible to directly assess the structural influence of troponin on tropomyosin that causes actin filaments, and hence myosin-crossbridge cycling and contraction, to switch on and off. A C-terminal domain of troponin I is thought to be intimately involved in inducing tropomyosin movement to an inhibitory position that blocks myosin-crossbridge interaction. Release of this regulatory, latching domain from actin after Ca²⁺ binding to TnC (the Ca²⁺ sensor of troponin that relieves inhibition) presumably allows tropomyosin movement away from the inhibitory position on actin, thus initiating contraction. However, the structural interactions of the regulatory domain of TnI (the “inhibitory” subunit of troponin) with tropomyosin and actin that cause tropomyosin movement are unknown, and thus, the regulatory process is not well defined. Here, thin filaments were labeled with an engineered construct representing C-terminal TnI, and then, 3D electron microscopy was used to resolve where troponin is anchored on actin-tropomyosin. Electron microscopy reconstruction showed how TnI binding to both actin and tropomyosin at low Ca²⁺ competes with tropomyosin for a common site on actin and drives tropomyosin movement to a constrained, relaxing position to inhibit myosin-crossbridge association. Thus, the observations reported reveal the structural mechanism responsible for troponin-tropomyosin-mediated steric interference of actin-myosin interaction that regulates muscle contraction.     

Effects of thin and thick filament proteins on calcium binding and exchange with cardiac troponin C

Understanding the effects of thin and thick filament proteins on the kinetics of Ca(2+) exchange with cardiac troponin C is essential to elucidating the Ca(2+)-dependent mechanisms controlling cardiac muscle contraction and relaxation. Unlike labeling of the endogenous Cys-84, labeling of cardiac troponin C at a novel engineered Cys-53 with 2-(4'-iodoacetamidoanilo)napthalene-6-sulfonic acid allowed us to accurately measure the rate of calcium dissociation from the regulatory domain of troponin C upon incorporation into the troponin complex. Neither tropomyosin nor actin alone affected the Ca(2+) binding properties of the troponin complex. However, addition of actin-tropomyosin to the troponin complex decreased the Ca(2+) sensitivity ( approximately 7.4-fold) and accelerated the rate of Ca(2+) dissociation from the regulatory domain of troponin C ( approximately 2.5-fold). Subsequent addition of myosin S1 to the reconstituted thin filaments (actin-tropomyosin-troponin) increased the Ca(2+) sensitivity ( approximately 6.2-fold) and decreased the rate of Ca(2+) dissociation from the regulatory domain of troponin C ( approximately 8.1-fold), which was completely reversed by ATP. Consistent with physiological data, replacement of cardiac troponin I with slow skeletal troponin I led to higher Ca(2+) sensitivities and slower Ca(2+) dissociation rates from troponin C in all the systems studied. Thus, both thin and thick filament proteins influence the ability of cardiac troponin C to sense and respond to Ca(2+). These results imply that both cross-bridge kinetics and Ca(2+) dissociation from troponin C work together to modulate the rate of cardiac muscle relaxation.     

Erythrocyte actin and spectrin. Interactions with muscle contractile and regulatory proteins

Actin and spectrin were isolated from washed red blood cell membranes. Spectrin bound and polymerized erythrocyte actin in the absence of potassium. Spectrin coated onto polystyrene latex particles bound 8–9 mol of erythrocyte actin per mol of spectrin when actin was in its depolymerized state. Spectrin enhanced the interaction of erythrocyte actin with muscle myosin as manifested by changes in Mg²⁺-ATPase activity. A similar enhancement also was observed with muscle α-actinin while muscle tropomyosin abolished these effects. The data suggest that spectrin may play the role of polymerizing factor as well as the anchoring site for erythrocyte actin just as α-actinin is the anchoring site for actin filaments in muscle and other non-muscle cells.     

Cardiac tropomyosin crystals and their interactions with troponin subunits

Crystals and paracrystals of bovine cardiac tropomyosin and their mixtures with different combinations of troponin subunits were examined in the electron microscope after negative staining. Although the cardiac proteins gave most of the same crystalline and paracrystalline patterns as observed previously with skeletal muscle tropomyosin and troponin, two important differences were noted. Cardiac troponin T was incapable of forming hexagonal networks with either skeletal or cardiac muscle tropomyosins, while skeletal troponin T readily associated in this manner with tropomyosins from either tissue source. Also the characteristic paracrystalline pattern seen with skeletal muscle tropomyosin, troponin T and troponin C only in the presence of calcium was consistently obtained with mixtures of the corresponding cardiac components when calcium was absent.     

THE EFFECT OF VARIOUS PROTEIN FRACTIONS ON Z- AND M-LINE RECONSTITUTION

Extraction of thin, glycerinated bundles of rabbit psoas muscle with a low ionic strength solvent results in removal first of M lines and then of Z lines. When these extracted myofibrillar bundles are allowed to interact, at adjusted ionic conditions, with the dilute myofibrillar extract or with the fractions obtained at 40% ammonium sulfate saturation from either the myofibrillar extract or from the Bailey extract of natural actomyosin, reconstitution of Z lines occurs. The ammonium sulfate fraction from the Bailey extract of natural actomyosin restores the tetragonal lattice structure of the Z line. Other structural features such as I-band tufts or cross-bridges, M lines and H-zone binding also occur with some of the proteins used for recombination. Although it has not yet been possible to identify exactly the protein(s) constituting the Z line, it appears unlikely that tropomyosin or troponin alone is the major protein of the Z line. A more likely candidate is α-actinin or a combination of α-actinin with another protein(s). In addition, this study demonstrates that basic morphological differences exist between cross-sections through the Z-line lattice and cross-sections through tropomyosin crystals.     

Pig platelet tropomyosin: interactions with the other thin-filament proteins

Pig platelet tropomyosin exhibits many of the functional activities of skeletal tropomyosin. At low ionic strength it forms end-to-end aggregates similar to those formed by skeletal tropomyosins. It forms a 1:1 complex with muscle troponin or with a troponin I-pig brain calmodulin complex, as well as a 1:6 association with platelet filamentous actin. Electron microscopy of paracrystals shows that the troponin binding site is slightly C-terminal of the unique cysteine, corresponding to position 190 of the rabbit skeletal alpha-tropomyosin sequence. The effect of a complex comprising platelet actin and tropomyosin on the ATPase activity of rabbit skeletal muscle myosin subfragment-1 was similar to that displayed by its skeletal muscle counterpart. Platelet tropomyosin decreased the activity by roughly half in a calcium-independent manner. Addition of troponin to the actin-tropomyosin in the absence of calcium results in further inhibition and allows the full activity of the complex to be restored by Ca2+. These results differ from those obtained by C?té & Smillie for horse platelet tropomyosin and this may reflect the different isomeric nature of pig platelet tropomyosin. These results suggest that the functional properties of non-muscle tropomyosins may differ when comparisons are made between proteins isolated from the same type of cell but in different species. Differences in self-association and actin-binding properties may be finely graded between different isoforms.     

Tropomyosin and troponin are required for ovarian contraction in the Caenorhabditis elegans reproductive system

Ovulation in the nematode Caenorhabditis elegans is coordinated by interactions between the somatic gonad and germ cells. Myoepithelial sheath cells of the proximal ovary are smooth muscle-like cells, but the regulatory mechanism of their contraction is unknown. We show that contraction of the ovarian muscle requires tropomyosin and troponin, which are generally major actin-linked regulators of contraction of striated muscle. RNA interference of tropomyosin or troponin C caused sterility by inhibiting ovarian contraction that is required for expelling mature oocytes into the spermatheca where fertilization takes place, thus causing accumulation of endomitotic oocytes in the ovary. Tropomyosin and troponin C were associated with actin filaments in the myoepithelial sheath, and the association of troponin C with actin was dependent on tropomyosin. A mutation in the actin depolymerizing factor/cofilin gene suppressed the ovulation defects by RNA interference of tropomyosin or troponin C. These results strongly suggest that tropomyosin and troponin are the actin-linked regulators for contraction of ovarian muscle in the C. elegans reproductive system.     

人骨骼肌α辅肌动蛋白（α－actinin）的提取及其抗血清的制备和鉴定

本文提取人骨骼肌α辅肌动蛋白（α－ａｃｔｉｎｉｎ）是综合了文献报导有关提取兔肌α－ａｃｔｉｎｉｎ的和提取鸡胗α－ａｃｔｉｎｉｎ的方法,稍加修改而确定的。用Ｈａｓｓｅｌｂａｃｈ－Ｓｃｈｎｅｉｄｅｒ缓冲液提取骨骼肌中的肌球蛋白后,将残余物经硼酸－缓冲液提取、匀浆及高速离心去掉肌动蛋白和肌原纤维的其它成份,上清加硫酸铵至３０％,３５％饱合度所得的沉淀用２２０ｍｍｏｌ／ＬＴｒｉｓ－乙酸溶解、透析、离心后经ＤＥ－５２柱层析可得电泳纯。α－ａｃｔｉｎｉｎ。将人骨骼肌α－ａｃｔｉｎｉｎ纯化制品免疫了三只大耳白纯种家兔,两个多月后,三只兔子都产生免抗人骨骼肌α－ａｃｔｉｎｉｎ的特异抗血清,用双向免疫扩散法和酶联免疫吸附试验（ＥＬＩＳＡ）测定,产生的抗体效价较高,用双扩散法测定效价为１：３２,用ＥＬＩＳＡ测定,用比率法判断结果,效价最高者为１：１００,０００左右,经免疫电镜观察结果证实,上述抗血清可以满足进一步实验要求。     

Troponin T core structure and the regulatory NH2-terminal variable region

The conserved central and COOH-terminal regions of troponin T (TnT) interact with troponin C, troponin I, and tropomyosin to regulate striated muscle contraction. Phylogenic data show that the NH2-terminal region has evolved as an addition to the conserved core structure of TnT. This NH2-terminal region does not bind other thin filament proteins, and its sequence is hypervariable between fiber type and developmental isoforms. Previous studies have demonstrated that NH2-terminal modifications alter the COOH-terminal conformation of TnT and thin filament Ca2+-activation, yet the functional core structure of TnT and the mechanism of NH2-terminal modulation are not well understood. To define the TnT core structure and investigate the regulatory role of the NH2-terminal variable region, we investigated two classes of model TnT molecules: (1) NH2-terminal truncated cardiac TnT and (2) chimera proteins consisting of an acidic or basic skeletal muscle TnT NH2-terminus spliced to the cardiac TnT core. Deletion of the TnT hypervariable NH2-terminus preserved binding to troponin I and tropomyosin and sustained cardiac muscle contraction in the heart of transgenic mice. Further deletion of the conserved central region diminished binding to tropomyosin. The reintroduction of differently charged NH2-terminal domains in the chimeric molecules produced long-range conformational changes in the central and COOH-terminal regions to alter troponin I and tropomyosin binding. Similar NH2-terminal charge effects are demonstrated in naturally occurring cardiac TnT isoforms, indicating a physiological significance. These results suggest that the hypervariable NH2-terminal region modulates the conformation and function of the TnT core structure to fine-tune muscle contractility.