A cytoskeletal tropomyosin can compromise the structural integrity of skeletal muscle

The α-arrestin SUP-13/ARRD-15 promotes isoform turnover of actin-interacting protein 1 in Caenorhabditis elegans striated muscle

Precise arrangement of actin, myosin, and other regulatory components in a sarcomeric pattern is critical for producing contractile forces in striated muscles. Actin-interacting protein 1 (AIP1), also known as WD-repeat protein 1 (WDR1), is one of essential factors that regulate sarcomeric assembly of actin filaments. In the nematode Caenorhabditis elegans, mutation in unc-78, encoding one of the two AIP1 isoforms, causes severe disorganization of sarcomeric actin filaments and near paralysis, but mutation in sup-13 suppresses the unc-78-mutant phenotypes to restore nearly normal sarcomeric actin organization and worm motility. Here, we identified that sup-13 is a nonsense allele of arrd-15 encoding an α-arrestin. The sup-13/arrd-15 mutation suppressed the phenotypes of unc-78 null mutant but required aipl-1 that encodes a second AIP1 isoform. aipl-1 was normally expressed highly in embryos and downregulated in mature muscle. However, in the sup-13/arrd-15 mutant, the AIPL-1 protein was maintained at high levels in adult muscle to compensate for the absence of the UNC-78 protein. The sup-13/arrd-15 mutation caused accumulation of ubiquitinated AIPL-1 protein, suggesting that a normal function of sup-13/arrd-15 is to enhance degradation of ubiquitinated AIPL-1, thereby promoting transition of AIP1 isoforms from AIPL-1 to UNC-78 in developing muscle. These results suggest that α-arrestin is a novel factor to promote isoform turnover by enhancing protein degradation.

Novel human cell expression method reveals the role and prevalence of posttranslational modification in nonmuscle tropomyosins

Biochemical studies require large quantities of proteins, which are typically obtained using bacterial overexpression. However, the folding machinery in bacteria is inadequate for expressing many mammalian proteins, which additionally undergo posttranslational modifications (PTMs) that bacteria, yeast, or insect cells cannot perform. Many proteins also require native N- and C-termini and cannot tolerate extra tag amino acids for proper function. Tropomyosin (Tpm), a coiled coil protein that decorates most actin filaments in cells, requires both native N- and C-termini and PTMs, specifically N-terminal acetylation (Nt-acetylation), to polymerize along actin filaments. Here, we describe a new method that combines native protein expression in human cells with an intein-based purification tag that can be precisely removed after purification. Using this method, we expressed several nonmuscle Tpm isoforms (Tpm1.6, Tpm1.7, Tpm2.1, Tpm3.1, Tpm3.2, and Tpm4.2) and the muscle isoform Tpm1.1. Proteomics analysis revealed that human-cell-expressed Tpms present various PTMs, including Nt-acetylation, Ser/Thr phosphorylation, Tyr phosphorylation, and Lys acetylation. Depending on the Tpm isoform (humans express up to 40 Tpm isoforms), Nt-acetylation occurs on either the initiator methionine or on the second residue after removal of the initiator methionine. Human-cell-expressed Tpms bind F-actin differently than their Escherichia coli-expressed counterparts, with or without N-terminal extensions intended to mimic Nt-acetylation, and they can form heterodimers in cells and in vitro. The expression method described here reveals previously unknown features of nonmuscle Tpms and can be used in future structural and biochemical studies with Tpms and other proteins, as shown here for α-synuclein.

Tropomyosin - master regulator of actin filament function in the cytoskeleton

Tropomyosin (Tpm) isoforms are the master regulators of the functions of individual actin filaments in fungi and metazoans. Tpms are coiled-coil parallel dimers that form a head-to-tail polymer along the length of actin filaments. Yeast only has two Tpm isoforms, whereas mammals have over 40. Each cytoskeletal actin filament contains a homopolymer of Tpm homodimers, resulting in a filament of uniform Tpm composition along its length. Evidence for this 'master regulator' role is based on four core sets of observation. First, spatially and functionally distinct actin filaments contain different Tpm isoforms, and recent data suggest that members of the formin family of actin filament nucleators can specify which Tpm isoform is added to the growing actin filament. Second, Tpms regulate whole-organism physiology in terms of morphogenesis, cell proliferation, vesicle trafficking, biomechanics, glucose metabolism and organ size in an isoform-specific manner. Third, Tpms achieve these functional outputs by regulating the interaction of actin filaments with myosin motors and actin-binding proteins in an isoform-specific manner. Last, the assembly of complex structures, such as stress fibers and podosomes involves the collaboration of multiple types of actin filament specified by their Tpm composition. This allows the cell to specify actin filament function in time and space by simply specifying their Tpm isoform composition.

Functional diversity of actin cytoskeleton in neurons and its regulation by tropomyosin

Neurons comprise functionally, molecularly, and spatially distinct subcellular compartments which include the soma, dendrites, axon, branches, dendritic spines, and growth cones. In this chapter, we detail the remarkable ability of the neuronal cytoskeleton to exquisitely regulate all these cytoplasmic distinct partitions, with particular emphasis on the microfilament system and its plethora of associated proteins. Importance will be given to the family of actin-associated proteins, tropomyosin, in defining distinct actin filament populations. The ability of tropomyosin isoforms to regulate the access of actin-binding proteins to the filaments is believed to define the structural diversity and dynamics of actin filaments and ultimately be responsible for the functional outcome of these filaments.

Identification of proteins interacting with lactate dehydrogenase in claw muscle of the porcelain crab Petrolisthes cinctipes

Biochemical adaptation of enzymes involves conservation of activity, stability and affinity across a wide range of intracellular and environmental conditions. Enzyme adaptation by alteration of primary structure is well known, but the roles of protein-protein interactions in enzyme adaptation are less well understood. Interspecific differences in thermal stability of lactate dehydrogenase (LDH) in porcelain crabs (genus Petrolisthes) are related to intrinsic differences among LDH molecules and by interactions with other stabilizing proteins. Here, we identified proteins that interact with LDH in porcelain crab claw muscle tissue using co-immunoprecipitation, and showed LDH exists in high molecular weight complexes using size exclusion chromatography and Western blot analyses. Co-immunoprecipitated proteins were separated using 2D SDS PAGE and analyzed by LC/ESI using peptide MS/MS. Peptide MS/MS ions were compared to an EST database for Petrolisthes cinctipes to identify proteins. Identified proteins included cytoskeletal elements, glycolytic enzymes, a phosphagen kinase, and the respiratory protein hemocyanin. Our results support the hypothesis that LDH interacts with glycolytic enzymes in a metabolon structured by cytoskeletal elements that may also include the enzyme for transfer of the adenylate charge in glycolytically produced ATP. Those interactions may play specific roles in biochemical adaptation of glycolytic enzymes.

Tropomyosin isoforms and reagents

Tropomyosins are rod-like dimers which form head-to-tail polymers along the length of actin filaments and regulate the access of actin binding proteins to the filaments.1 The diversity of tropomyosin isoforms, over 40 in mammals, and their role in an increasing number of biological processes presents a challenge both to experienced researchers and those new to this field. The increased appreciation that the role of these isoforms expands beyond that of simply stabilizing actin filaments has lead to a surge of reagents and techniques to study their function and mechanisms of action. This report is designed to provide a basic guide to the genes and proteins and the availability of reagents which allow effective study of this family of proteins. We highlight the value of combining multiple techniques to better evaluate the function of different tm isoforms and discuss the limitations of selected reagents. Brief background material is included to demystify some of the unfortunate complexity regarding this multi-gene family of proteins including the unconventional nomenclature of the isoforms and the evolutionary relationships of isoforms between species. Additionally, we present step-by-step detailed experimental protocols used in our laboratory to assist new comers to the field and experts alike.

Internal and external paralogy in the evolution of tropomyosin genes in metazoans

Nature contains a tremendous diversity of forms both at the organismal and genomic levels. This diversity motivates the twin central questions of molecular evolution: what are the molecular mechanisms of adaptation, and what are the functional consequences of genomic diversity. We report a 22-species comparative analysis of tropomyosin (PPM) genes, which exist in a variety of forms and are implicated in the emergence of a wealth of cellular functions, including the novel muscle functions integral to the functional diversification of bilateral animals. TPM genes encode either or both of long-form [284 amino acid (aa)] and short-form (approximately 248 aa) proteins. Consistent with a role of TPM diversification in the origins and radiation of bilaterians, we find evidence that the muscle-specific long-form protein arose in proximal bilaterian ancestors (the bilaterian 'stem'). Duplication of the 5' end of the gene led to alternative promoters encoding long- and short-form transcripts with distinct functions. This dual-function gene then underwent strikingly parallel evolution in different bilaterian lineages. In each case, recurrent tandem exon duplication and mutually exclusive alternative splicing of the duplicates, with further association between these alternatively spliced exons along the gene, led to long- and short-form-specific exons, allowing for gradual emergence of alternative "internal paralogs" within the same gene. We term these Mutually exclusively Alternatively spliced Tandemly duplicated Exon sets "MATEs". This emergence of internal paralogs in various bilaterians has employed every single TPM exon in at least one lineage and reaches striking levels of divergence with up to 77% of long- and short-form transcripts being transcribed from different genomic regions. Interestingly, in some lineages, these internal alternatively spliced paralogs have subsequently been "externalized" by full gene duplication and reciprocal retention/loss of the two transcript isoforms, a particularly clear case of evolution by subfunctionalization. This parallel evolution of TPM genes in diverse metazoans attests to common selective forces driving divergence of different gene transcripts and represents a striking case of emergence of evolutionary novelty by alternative splicing.

New insights into the regulation of the actin cytoskeleton by tropomyosin

The actin cytoskeleton is regulated by a variety of actin-binding proteins including those constituting the tropomyosin family. Tropomyosins are coiled-coil dimers that bind along the length of actin filaments. In muscles, tropomyosin regulates the interaction of actin-containing thin filaments with myosin-containing thick filaments to allow contraction. In nonmuscle cells where multiple tropomyosin isoforms are expressed, tropomyosins participate in a number of cellular events involving the cytoskeleton. This chapter reviews the current state of the literature regarding tropomyosin structure and function and discusses the evidence that tropomyosins play a role in regulating actin assembly.

Diverse roles of the actin cytoskeleton in striated muscle

In addition to the highly specialized contractile apparatus, it is becoming increasingly clear that there is an extensive actin cytoskeleton which underpins a wide range of functions in striated muscle. Isoforms of cytoskeletal actin and actin-associated proteins (non-muscle myosins, cytoskeletal tropomyosins, and cytoskeletal alpha-actinins) have been detected in a number of regions of striated muscle: the sub-sarcolemmal costamere, the Z-disc and the T-tubule/sarcoplasmic reticulum membranes. As the only known function of these proteins is through association with actin filaments, their presence in striated muscles indicates that there are spatially and functionally distinct cytoskeletal actin filament systems in these tissues. These filaments are likely to have important roles in mechanical support, ion channel function, myofibrillogenenous and vesicle trafficking.