thought that the function and form of the actin cytoskeleton is controlled largely by actin-binding proteins (ABP) that are associated with the actin polymer. ABP are a group of proteins that bind to actin. Although tropomyosin is sometimes included as an ABP, it is not a true ABP. The tropomyosin dimer has very low affinity for an actin filament and forms no van der waals contacts with actin. It is only the formation of a tropomyosin polymer's winding around the actin filament that provides stability to the tropomyosin-actin filament interaction. Many studies suggest that the binding of tropomyosin isoforms to an actin filament may influence the binding of other ABPs, which together alter the structure and convey specific properties and ultimately specific functions to an actin filament. This is demonstrated in neuroepithelial cells, where increased expression of Tropomyosin 5NM1 increases the recruitment of myosin IIB, a myosin motor protein to the growth cone area. However, the over-expression of Tropomyosin Br3 had the opposite effect, decreasing myosin activity in the same region. In a pioneering study by Bernstein and Bamburg, it was observed that the actin-binding protein actin depolymerisation factor (ADF)/cofilin, a factor that promotes actin filament depolymerisation, competed with tropomyosin for binding to the actin filament. The expression of Tropomyosin 5NM1 in neuronal cells eliminated ADF/cofilin from the growth cone region, leading to more stable actin filaments. However, the increased expression of Tropomyosin Br3 was observed to recruit ADF/cofilin to actin filaments bound to the Tropomyosin Br3 isoform within the lamellipodium, which led to the disassembly of actin filaments. This phenomenon, whereby a specific tropomyosin isoform directs specific interactions between actin-binding proteins, and the actin filament has been observed in a variety of model systems with a range of different binding proteins (reviewed in Gunning et al., 2008). These interactions, under the influence of tropomyosin isoforms, allow actin filaments to be involved in a diverse range of cellular functions. Function in skeletal muscle contraction Skeletal muscle is composed of large, multi-nucleated cells (muscle fibers). Each muscle fiber is packed with longitudinal arrays of myofibrils. Myofibrils are composed of repeating protein structures or sarcomeres, the basic functional unit of skeletal muscle. The sarcomere is a highly structured protein array, consisting of interdigitating thick and thin filaments, where the thin filaments are tethered to a protein structure, the Z-line. The dynamic interaction between the thick and thin filaments results in muscle contraction.