Abstract
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.
Highlights
Tropomyosin (Tpm) is best known for its role in the regulation of contraction of skeletal muscle and the heart
The actin thin filament is composed of three core elements: a double-stranded polymer of actin, two continuous polymers of Tpm running along each side of the actin and the troponin complex, a heteromeric protein complex consisting of troponin T (TnT), troponin I (TnI) and troponin C (TnC), which is located on each Tpm dimer (Lehman and Craig, 2008)
We focus on four main issues: (i) the mechanism of assembly of actin filaments that contain homopolymers of specific Tpm isoforms, (ii) the physiological function of specific populations of actin filaments that contain different Tpms, (iii) the mechanism by which different Tpms direct different functional outcomes and (iv) how different Tpm-containing filaments contribute to the assembly of large-scale actin-based structures
Summary
Tropomyosin (Tpm) is best known for its role in the regulation of contraction of skeletal muscle and the heart. In these cells, a mechanism must exist to regulate which Tpm binds to the growing actin polymer in order to define Tpm specificity, thereby modulating the functional properties of the actin filament.
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