Tropomyosin Function Research Articles

Subcellular localization and possible function of actin, tropomyosin and actin-related protein 3 (Arp3) in the fission yeast Schizosaccharomyces pombe

We investigated subcellular localizations and interactions of actin and two actin cytoskeleton-related proteins, Cdc8 tropomyosin and actin-related protein 3, Arp3, in the fission yeast Schizosaccharomyces pombe, using specific antibodies and by gene disruption. Actin was localized to the medial microfilamentous ring in the region of the septum during cytokinesis and to cortical patches by immunoelectron microscopy. F-actin cables were detected throughout the cell cycle by fluorescent staining with Bodipy-phallacidin. Cables were often linked to the patches and to the medial ring during its formation. Tropomyosin was localized to the medial ring and the cables. It was also distributed in the cell as patches, although co-localization with F-actin was not frequent. In cdc8ts mutant cells, F-actin cables were not observed although the F-actin patches were detected and cell polarity was maintained. These observations suggest that the F-actin cables may be involved in the formation of the medial ring, and that tropomyosin plays an important role in organizing both the ring and the cable, but is not involved in the F-actin patch formation or maintenance of cell polarity. Binding of Arp3 to actin was revealed by immunoprecipitation as well as by DNase I column chromatography. Arp3 seemed to form a complex with several proteins in the cell extracts, as previously reported for other organisms. Contrary to a previous report (McCollum et al., EMBO J. 15, 6438-6446, 1996), Arp3 was found to be concentrated in the medial region from early anaphase to late cytokinesis. Following arp3 gene disruption, F-actin patches were delocalized throughout the cell and cells did not undergo polarized growth, suggesting that Arp3 influences the proper localization of the actin patches in the cell and thereby controls the polarized growth of the cell.

SRO9, a multicopy suppressor of the bud growth defect in the Saccharomyces cerevisiae rho3-deficient cells, shows strong genetic interactions with tropomyosin genes, suggesting its role in organization of the actin cytoskeleton.

RHO3 encodes a Rho-type small GTPase in the yeast Saccharomyces cerevisiae and is involved in the proper organization of the actin cytoskeleton required for bud growth. SRO9 (YCL37c) was isolated as a multicopy suppressor of a rho3delta mutation. An Sro9p domain required for function is similar to a domain in the La protein (an RNA-binding protein). Disruption of SRO9 did not affect vegetative growth, even with the simultaneous disruption of an SRO9 homologue, SRO99. However, sro9delta was synthetically lethal with a disruption of TPM1, which encodes tropomyosin; sro9delta tpm1delta cells did not distribute cortical actin patches properly and lysed. We isolated TPM2, the other gene for tropomyosin, as a multicopy suppressor of a tpm1delta sro9delta double mutant. Genetic analysis suggests that TPM2 is functionally related to TPM1 and that tropomyosin is important but not essential for cell growth. Overexpression of SRO9 suppressed the growth defect in tpm1delta tpm2delta cells, disappearance of cables of actin filaments in both rho3delta cells and tpm1delta cells, and temperature sensitivity of actin mutant cells (act1-1 cells), suggesting that Sro9p has a function that overlaps or is related to tropomyosin function. Unlike tropomyosin, Sro9p does not colocalize with actin cables but is diffusely cytoplasmic. These results suggest that Sro9p is a new cytoplasmic factor involved in the organization of actin filaments.

The Sequence of the Alternatively Spliced Sixth Exon of α-Tropomyosin Is Critical for Cooperative Actin Binding but Not for Interaction with Troponin

Tropomyosins, a family of highly conserved coiled-coil actin binding proteins, can differ as a consequence of alternative expression of several exons (Lees-Miller, J., and Helfman, D. (1991) BioEssays 13, 429-437). Exon 6, which encodes residues 189-213 in long, 284-residue tropomyosins, has two alternative forms, exon 6a or 6b, both highly conserved throughout evolution. In alpha-tropomyosin, exon 6a or 6b is not specific to any one of the nine isoforms. Exon 6b encodes part of a putative Ca2+-sensitive troponin binding site in striated muscle tropomyosins, suggesting that the exon 6-encoded region may be specialized for certain tropomyosin functions. A series of recombinant, unacetylated tropomyosin exon 6 deletion and substitution mutants and chimeras was expressed in Escherichia coli to determine the requirements of exon 6 for tropomyosin function. Functional properties of the tropomyosins were defined by actin affinity measured by cosedimentation, troponin T affinity using a newly developed biosensor assay, and regulation of the actomyosin MgATPase. The region of tropomyosin encoded by exon 6 affects actin affinity but not thin filament assembly, troponin T binding, or regulation with troponin. The tropomyosins with exon 6a or 6b function normally whether a striated muscle exon 9a or smooth/non-muscle exon 9d is present. However, the effect of deleting 21 amino acids encoded by exon 6 or replacing it with a GCN4 leucine zipper sequence depends on the COOH-terminal sequence.

Requirement of amino-terminal modification for striated muscle alpha-tropomyosin function.

Striated muscle alpha-tropomyosin expressed in Escherichia coli is unacetylated, polymerizes poorly, and binds weakly to F-actin (Hitchcock-DeGregori, S. E., and Heald, R. W. (1987) J. Biol. Chem. 262, 9730-9735). To define the structural requirements of NH2-terminal modification for striated tropomyosin function, an acetylated recombinant tropomyosin and an unacetylated short fusion recombinant tropomyosin were compared. An acetylated recombinant chicken striated muscle alpha-tropomyosin was expressed in insect Sf9 cells using the baculovirus expression vector system. The purified tropomyosin (approximately 15 mg/liter of insect cell suspension) polymerized, comigrated with chicken striated alpha-tropomyosin purified from muscle on two-dimensional polyacrylamide gels, was blocked at the NH2 terminus, and had the same actin affinity as muscle tropomyosin. These results conclusively show the importance of NH2-terminal acetylation for striated tropomyosin function. To learn if a short fusion peptide would substitute for amino-terminal acetylation, tropomyosin with AlaSer-Arg on the NH2 terminus was constructed and expressed in E. coli as an unacetylated protein. This f3-tropomyosin bound to actin with a 10-fold higher affinity than striated muscle alpha-TM and, unlike muscle tropomyosin, exhibited a shear-dependent viscosity. The altered function of f3-tropomyosin shows that the naturally occurring acetylated NH2 terminus is required for full, normal function. It is proposed that a major requirement for cooperative binding of striated muscle tropomyosin to actin is modification of the alpha-amino group of methionine to be an amide, as when acetylated or in a peptide bond in a fusion protein, to make the extreme NH2 terminus more hydrophobic. The results are discussed in terms of known coiled coil structure.

Structural requirements of tropomyosin for binding to filamentous actin.

Tropomyosin is an actin binding protein found in virtually all eucaryotic cells. Since its early discovery (Bailey, 1948), the actin binding properties and regulatory functions of tropomyosin have been extensively investigated. In addition, tropomyosin has served as a prototype for the structure of α-helical coiled-coil proteins (Cohen and Parry, 1990). The recent recognition of the diversity of tropomyosins in different cell types and discovery of the relationship between cell shape and isoform expression are indicative of a fundamental role for tropomyosin in the actin cytoskeleton. There is a need for a better understanding of structure-function relationships in this protein.

The amino terminus of muscle tropomyosin is a major determinant for function.

The amino-terminal region of muscle tropomyosin is highly conserved among muscle and 284-residue non-muscle tropomyosins. Analysis of fusion and nonfusion striated alpha-tropomyosins and a mutant in which residues 1-9 have been deleted has shown that the amino terminus is crucial for function. The presence of 80 amino acids of a nonstructural influenza virus protein (NS1) on the amino terminus of tropomyosin allows magnesium-independent binding of tropomyosin to actin. The fusion tropomyosin inhibits the actomyosin S1 ATPase at all myosin S1 concentrations tested, indicating that the presence of the fusion peptide prevents myosin S1 from switching the actin filament from the inhibited to the potentiated state. Nonfusion tropomyosin, an unacetylated form, has no effect on the actomyosin S1 ATPase, though it regulates normally with troponin. Deletion of residues 1-9, which are believed to overlap with the carboxyl-terminal end of tropomyosin in the thin filament, results in loss of tropomyosin function. The mutant is unable to bind to actin, in the presence and absence of troponin, and it has no regulatory function. The removal of the first 9 residues of tropomyosin is much more deleterious than removal of the last 11 by carboxypeptidase digestion. We suggest that the structure of the amino-terminal region and acetylation of the initial methionine are crucial for tropomyosin function.

Probing the role of nonmuscle tropomyosin isoforms in intracellular granule movement by microinjection of monoclonal antibodies.

Chicken embryo fibroblast (CEF) cells were microinjected with several different monoclonal antibodies that recognize certain nonmuscle isoforms of tropomyosin. Immediately after injection, cells were recorded with a time-lapse video imaging system; later analysis of the tapes revealed that particles in cells injected with one of these antibodies (CG1, specific for CEF tropomyosin isoforms 1 and 3) showed a dramatic decrease in instantaneous speed while moving, distance moved per saltation, and proportion of time spent in motion. Injection of Fab fragments of CG1 resulted in similar changes in the pattern of granule movement. This inhibition of granule movement by CG1 antibody was reversible; at 2.5 h after injection, granules in injected cells had already reached three-fourths of normal speed. The speed of granule movement in cells injected either with antibody specific for tropomyosin isoforms not present in CEF cells, or with CG1 antibody preabsorbed with tropomyosin, was not significantly different from the speed of granules in uninjected cells. When cells were injected with CG1 or Fab fragments of CG1, fixed, and counter-stained with rabbit antibodies to reveal the microtubule, microfilament, and intermediate filament systems, no obvious differences from the patterns normally seen in uninjected cells were observed. Examination of the ultrastructure of injected cells by EM confirmed the presence of apparently intact and normal microtubule, actin, and intermediate filament networks. These experiments suggest that tropomyosin may play an important role in the movement of vesicles and organelles in the cell cytoplasm. Also, we have shown previously that the CG1 determinant can undergo a motility-dependent change in reactivity, that may be important for the regulatory function of nonmuscle tropomyosin (Hegmann, T. E., J. L.-C. Lin, and J. J.-C. Lin. 1988. J. Cell Biol. 106:385-393). Therefore, in addition to postulated microtubule-based motors, microfilaments may play a critical role in regulating granule movement in nonmuscle cells.

The structure of the amino terminus of tropomyosin is critical for binding to actin in the absence and presence of troponin.

Analysis of two recombinant variants of chicken striated muscle alpha-tropomyosin has shown that the structure of the amino terminus is crucial for most aspects of tropomyosin function: affinity to actin, promotion of binding to actin by troponin, and regulation of the actomyosin MgATPase. Initial characterization of variants expressed and isolated from Escherichia coli has been published (Hitchcock-DeGregori, S. E., and Heald, R. W. (1987) J. Biol. Chem. 262, 9730-9735). Fusion tropomyosin contains 80 amino acids of a nonstructural influenza virus protein (NS1) on the amino terminus. Nonfusion tropomyosin is a variant because the amino-terminal methionine is not acetylated (unacetylated tropomyosin). The affinity of tropomyosin labeled at Cys190 with N-[14C]ethylmaleimide for actin was measured by cosedimentation in a Beckman Airfuge. Fusion tropomyosin binds to actin with an affinity slightly greater than that of chicken striated muscle alpha-tropomyosin (Kapp = 1-2 X 10(7) versus 0.5-1 X 10(7) M-1) and more strongly than unacetylated tropomyosin (Kapp = 3 X 10(5) M-1). Both variants bind cooperatively to actin. Troponin increases the affinity of unacetylated tropomyosin for actin (+Ca2+, Kapp = 6 X 10(6) M-1; +EGTA, Kapp = 2 X 10(7) M-1), but the affinity is still lower than that of muscle tropomyosin for actin in the presence of troponin (Kapp much greater than 10(8) M-1). Troponin has no effect on the affinity of fusion tropomyosin for actin indicating that binding of troponin T to the over-lap region of the adjacent tropomyosin, presumably sterically prevented by the fusion peptide in fusion tropomyosin, is required for troponin to promote the binding of tropomyosin to actin. The role of troponin T in regulation and the mechanisms of cooperative binding of tropomyosin to actin have been discussed in relation to this work.

Altered actin and troponin binding of amino-terminal variants of chicken striated muscle alpha-tropomyosin expressed in Escherichia coli.

We have expressed two variants of chicken striated muscle alpha-tropomyosin in Escherichia coli: fusion tropomyosin containing 80 amino acids of a non-structural influenza virus protein (NS1) on the amino terminus and a non-fusion tropomyosin which is a variant because the amino-terminal methionine is not acetylated (unacetylated tropomyosin). From our analysis of purified proteins in vitro we suggest that the amino-terminal region, which is highly conserved in muscle tropomyosins, is crucial for all aspects of tropomyosin function. Both forms are altered in tropomyosin activity: neither shows head-to-tail polymerization, with or without troponin. Unacetylated tropomyosin binds weakly to actin, but in the presence of troponin it binds well and can regulate the actomyosin ATPase. Fusion tropomyosin binds well to actin, but binding of troponin is calcium-sensitive and it does not confer effective calcium sensitivity on the actomyosin ATPase. Our results indicate that the local charge at the amino terminus is critical for actin binding but that normal head-to-tail association is not required. The properties of fusion tropomyosin-troponin interaction are indicative of impaired troponin T binding to tropomyosin and provide evidence for its binding to the amino terminus of tropomyosin.

Purification and characterization of multiple isoforms of tropomyosin from rat cultured cells.

We have previously shown that rat cultured cells contain five isoforms of tropomyosin (Matsumura, F., Yamashiro-Matsumura, S., and Lin, J. J.-C. (1983) J. Biol. Chem. 258, 6636-6644) and that these tropomyosins are differentially expressed upon cell transformation (Matsumura, F., Lin, J. J.-C., Yamashiro-Matsumura, S., Thomas, G. P., and Topp, W. C. (1983) J. Biol. Chem. 258, 13954-13964). To examine functions of tropomyosin in microfilament organization, we have purified and partially separated the multiple isoforms of tropomyosin by chromatography on hydroxylapatite. Analyses of cross-linked dimers produced by air oxidation have revealed that all isoforms except the tropomyosin isoform with apparent Mr of 35,000 form homodimers. Although these tropomyosins share many properties characteristic of tropomyosin, structural analyses at a peptide level and immunological analyses have shown that the five isoforms can be classified into two groups, i.e. tropomyosins with higher apparent Mr (Mr = 40,000, 36,500, and 35,000) and tropomyosins with lower apparent Mr (Mr = 32,400 and 32,000). The low Mr tropomyosins show less ability for head-to-tail polymerization and lower affinity to actin than the high Mr tropomyosins. We suggest that these differences in properties may be related to the changes in microfilament organization observed in transformed cells.

Alternative Splicing of a Drosophila Tropomyosin Gene Generates Muscle Tropomyosin Isoforms with Different Carboxy-Terminal Ends

The muscle tropomyosin I (mTm I) gene from Drosophila melanogaster has been analyzed and shown to express a complex transcription unit consisting of two sets of tissue-specific mRNAs. A 1.3- and 1.6-kilobase set of mRNAs is expressed during myogenesis in embryos, and in myogenic cell cultures. The mRNAs encode a 34,000-dalton muscle tropomyosin isoform. The same mTm I gene expresses a different set of 1.7- and 1.9-kilobase mRNAs in thoracic flight muscle tissue of the adult. The thorax RNAs encode a new tropomyosin isoform resolved on two-dimensional gels. The structure of the gene has been determined, and we show that the embryonic and thoracic mRNAs are generated by alternative splicing. The alternate exon splicing patterns determine a different 27 amino acids at the carboxy-terminal end of the two tropomyosin isoforms. These results show that the carboxy-terminal domain of tropomyosin is highly regulated in determining tropomyosin function. The results also show that contractile protein isoforms can be generated by single as well as multiple genes.

Alternative splicing of a Drosophila tropomyosin gene generates muscle tropomyosin isoforms with different carboxy-terminal ends.

The muscle tropomyosin I (mTm I) gene from Drosophila melanogaster has been analyzed and shown to express a complex transcription unit consisting of two sets of tissue-specific mRNAs. A 1.3- and 1.6-kilobase set of mRNAs is expressed during myogenesis in embryos, and in myogenic cell cultures. The mRNAs encode a 34,000-dalton muscle tropomyosin isoform. The same mTm I gene expresses a different set of 1.7- and 1.9-kilobase mRNAs in thoracic flight muscle tissue of the adult. The thorax RNAs encode a new tropomyosin isoform resolved on two-dimensional gels. The structure of the gene has been determined, and we show that the embryonic and thoracic mRNAs are generated by alternative splicing. The alternate exon splicing patterns determine a different 27 amino acids at the carboxy-terminal end of the two tropomyosin isoforms. These results show that the carboxy-terminal domain of tropomyosin is highly regulated in determining tropomyosin function. The results also show that contractile protein isoforms can be generated by single as well as multiple genes.

Tropomyosin: Crystal structure, polymorphism and molecular interactions

The α-protein tropomyosin forms a variety of ordered aggregates. True crystals with a very open lattice and at least two other net forms are produced near the isoelectric point together with fibrous aggregates. Three distinctive types of tactoids with axial periodicities about 400 Å are formed with divalent cations. The electron microscope observations on the polymorphic forms have been related to X-ray diffraction measurements on the crystal lattice and the tactoids produced with magnesium. The X-ray pattern of one projection of the crystal has been interpreted at low resolution from a model composed of rod-shaped molecules arranged in accord with the electron micrographs. The tropomyosin molecules in the crystal are associated head-to-tail in polar filaments with a 400 Å period. The filaments are periodically bent as a consequence of the cross connections at two sites alternatively separated by about 230 Å and 170 Å in the 400 Å axial period. The principal conclusions of this study are: there is a specific polar end-to-end bonding of tropomyosin molecules which defines the period of about 400 Å observed in the polymorphic nets and tactoids; there are two cross-connecting sites which are involved in net formation; in many of the forms, the polar filaments are arranged in oppositely directed pairs; the molecular coiled-coil is often supercoiled in the polar filaments. The periodicity in the I band of muscle can be identified with the 400 Å repeat characteristic of the end-to-end association of tropomyosin. The polymorphism observed in vitro may be related to the structural and regulatory functions of tropomyosin in muscle.

The Role of the Sulfhydryl Groups of Tropomyosin and Troponin in the Calcium Control of Actomyosin Contractility

Abstract Methods for preparing troponin and tropomyosin from ethanol-extracted muscle are presented. Examination of these proteins by disc electrophoresis indicated that the troponin preparation is homogeneous. If precautions were taken to assure that the —SH groups of tropomyosin were not permitted to oxidize during preparation or electrophoresis, tropomyosin appeared to be nearly homogeneous. Traces of troponin, however, appeared in all samples. Neither troponin nor tropomyosin was found to make actomyosin superprecipitation sensitive to the removal of Ca++. Combined, they were effective. A moderately wide range of troponin to tropomyosin ratios (between 1:1 and 3:1) were found to provide optimal sensitization of actomyosin. At a troponin to tropomyosin ratio of 1.3:1, 35 µg of this protein mixture inhibited by 85% 200 µg of actomyosin. The ability of troponin to collaborate with tropomyosin in the sensitization of actomyosin was lost to varying degrees if the troponin —SH groups were allowed to react with p-chloromercuribenzoate or N-ethylmaleimide or if the —SH groups were not protected during preparation. In contrast, the function of tropomyosin was found not to correlate with the state of its —SH groups. Troponin was found to be a potent calcium-binding substance, possessing 24 µmoles of high affinity calcium-binding sites per g. The high affinity site had an apparent affinity constant of approximately 2.4 x 106 m-1. It is hypothesized that the binding of calcium to this site inactivates troponin and that this is the mechanism whereby calcium activates myofibrillar contraction. It is concluded that tropomyosin does not possess a similar class of calcium-binding sites. The affinity of troponin for calcium was unaltered by reaction of its —SH groups. The number of high affinity calcium-binding sites was, however, doubled to approximately 40 µmoles of sites per g by reaction of troponin with —SH reagents.