Stress fiber

Stress Fiber
Stress fibers - visualized via fluorescence micrograph of F-actin
TH	H1.00.01.1.02033
Anatomical terminology []

Stress fibers are contractile actin bundles found in non-muscle cells. They are composed of actin (microfilaments) and non-muscle myosin II (NMMII), and also contain various crosslinking proteins, such as α-actinin, to form a highly regulated actomyosin structure within non-muscle cells. Stress fibers have 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 actin and myosin. Actin is a ~43kDa globular protein, and can polymerize to form long filamentous structures.These filaments are made of two strands of actin monomers (or protofilaments) wrapping around each other, to create a single actin filament. Because actin monomers are not symmetrical molecules, their filaments have polarity based upon the structure of the actin monomer, which will allow one end of the actin filament to polymerize faster than the other. The end that can polymerize faster is known as the plus-end, whereas the end that polymerizes slower is known as the minus-end. Stress fibers are usually composed of 10-30 actin filaments. Stress fibers are composed of antiparallel microfilaments: actin filaments are bundled along their length, and plus-ends and minus-ends co-mingle at each end of the bundle. The antiparallel arrangement of actin filaments within stress fibers is reinforced by α-actinin, an actin filament crosslinking protein which contains antiparallel actin-binding domains. These bundles are then cross-linked by NMMII to form stress fibers.

The Rho family of GTPases regulate many aspects of actin cytoskeletal dynamics, including stress fiber formation. RhoA (sometimes referred to as just 'Rho') is responsible for the formation of stress fibers, and its activity in stress fiber formation was first discovered by Ridley and Hall in 1992. When bound to GTP, Rho activates Rho-associated coiled-coil forming kinase (ROCK) and mammalian homologue of Drosophila diaphanous (mDia). mDia is a formin, which nucleates and polymerizes long actin filaments. ROCK is a kinase that acts to phosphorylate MLCP (myosin-light-chain phosphatase), as well as the NMMII light chain, which inactivates MLCP and activates myosin. This will lead to the accumulation of activated myosin motor proteins, which bind the actin filaments that were polymerized by mDia, to create stress fibers. In addition, ROCK also phosphorylates and activates LIM-kinase. LIM-kinase will in turn phosphorylate and inactivate cofilin, which will prevent the breakdown and recycling of actin filaments, maintaining the integrity of the stress fibers.