Thin filament proteins structure

Actin occurs in multiple isoforms within individual smooth muscle cells. All of these isoforms are capable of forming filamentous actin that can interact with myosin to generate force. Although the functional importance of the different actin isoforms is presently unclear, there is evidence to suggest that they may serve to "customize actin filaments to serve different functional roles within the cell by determining its interactions with different binding proteins. The first part of this section will review the molecular structure of the thin filament. The structure of actin and the relationship to the other protein constituents of the thin filament to actin... 

Cohen DM, Murphy RA (1979) Cellular thin filament protein contents and force generation in porcine arteries and veins. Circ Res 45 661-665 Cohen P (1989) The structure and regulation of protein phosphatases. Annu Rev Biochem 58 453-508... 

Tropomyosin and troponin are proteins located in the thin filaments, and together with Ca2+, they regulate the interaction of actin and myosin (Fig. 43-3) [5]. Tropomyosin is an a-helical protein consisting of two polypeptide chains its structure is similar to that of the rod portion of myosin. Troponin is a complex of three proteins. If the tropomyosin-troponin complex is present, actin cannot stimulate the ATPase activity of myosin unless the concentration of free Ca2+ increases substantially, while a system consisting solely of purified actin and myosin does not exhibit any Ca2+ dependence. Thus, the actin-myosin interaction is controlled by Ca2+ in the presence of the regulatory troponin-tropomyosin complex [6]. 

The above processes describe how the growth and depolymerisation of actin filaments thin filaments) is controlled. However, actin filaments are assembled into filamentous networks, and these three-dimensional structures are themselves controlled and also stabilised by a number of proteins ... 

Until the past decade, the cytoplasm was widely considered to be structurally unorganized with the main division of labor at the organellar level. Certainly, relatively little was known about the nature of the cyto-skeleton (with the notable exception of the mitotic apparatus and striated muscle), and the dynamics of cytoplasmic behavior were conceptualized vaguely in terms of sol-gel transitions without a sound molecular foundation. Substantial improvements in electron, light, and fluorescence microscopy, as well as the isolation of discrete protein components of the cytoskeleton, have led the way to a much better appreciation of the structural organization of the cytoplasm. Indeed, the lacelike network of thin filaments, intermediate filaments, and microtubules in nonmuscle cells is as familiar today as the organelles identified... 

As in the case of the myosin head, knowledge of actin filament structure, or thin filament structure as it is termed when tropomyosin and troponin are present, also progressed rapidly when the structure of the globular actin (G-actin) monomer was determined by protein crystallography in... 

Each myofibril consists of two different protein structures myofilaments. These are myosin thick (15 nm x 1.5 pm) and thin (7 nm x 1 pm) filaments made from actin, tropomyosin, and troponin. 

In vitro, o -actinin and filamin bind actin, the major protein of thin filaments. One a-actinin molecule binds to two actin filaments, one from each side of the Z-disk. Four a-actinin molecules bind to each actin filament at 90° angles, so that the actin filaments are bound into the Z-disks in a square array, although in most of the length of the sarcomere their arrangement is hexagonal. Desmin, an intermediate filament protein, forms a network from one Z-disk to the next across the myofibril. Such links, aided by attachment of desmin and dystrophin to the sarcolemma, help hold the sarcomere structure in... 

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