Stress fibers in a chicken fibroblast.
Stress fibers are higher order structures in non-muscle cells which consist of actin filaments (aka microfilaments), crosslinking proteins (proteins that bind two or more filaments together), such as α-actinins ACTN1 and ACTN4, and myosin II motor proteins. Originally thought to arise from the effects of tension, stress fibers have since been shown to play an important role in cellular contractility, providing force for a number of functions such as cell adhesion, migration and morphogenesis.
Stress fibers are primarily composed of 10 - 30 actin filaments. Actin is a globular protein (~43 kDa) which polymerizes to form a filamentous structure which has two actin protofilaments wrapping around each other. This forms a single 'actin filament' (previously referred to as 'microfilament'). The fiber may show alternating polarity, although this is not necessary and more complex configurations are possible. The orientation of the fibers help determine their contractile properties.
The myosin motors in the stress fibers move, sliding actin filaments past one another, so the fiber can contract. The process of stress fiber dynamics is currently being rigorously studied and is not yet well understood. In order for contraction to generate forces the fibers must be anchored on something. Stress fibers can anchor to the cell membrane, and frequently the sites where this anchoring occurs are also connected to structures outside the cell (the extracellular matrix or some other substrate). These connection sites are called focal adhesions. Many proteins are required for proper focal adhesion production and maintenance. Contraction against these fixed external substrates is what allows the force generated by myosin motors and filament growth and rearrangement to move and reshape the cell.
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