microfilament (actin filament)
/ MY-kroh-FIL-uh-ment /
Think of a long necklace made by stringing identical beads together, two such strands twisted gently around each other like a rope. That is the basic shape of a microfilament: two chains of a small protein called actin wound into a thin, flexible cable. Of the cytoskeleton's three fiber types, microfilaments are the thinnest, but they are everywhere just beneath the cell's surface, where they help set the shape of the cell membrane.
Each bead is a single actin protein (called G-actin, for globular). When many of them link up, they form a filament (F-actin, for filamentous). The two ends of the filament are different: one end (the plus or barbed end) tends to grow by adding new actin beads, while the other (the minus or pointed end) tends to lose them. This built-in lopsidedness lets the cell push membranes outward by growing actin at the front, and it gives the motor protein myosin a directional track to walk along.
Microfilaments are central to cell shape and short-range movement. They form the contractile ring that pinches a dividing cell in two, the core of the tiny finger-like microvilli that increase a cell's surface area, and the pushing engine behind crawling cells. In muscle, ordered arrays of actin and myosin produce the force of every heartbeat and every step you take. So although each filament is delicate, together they generate much of the mechanical work a cell can do.
When you cut yourself, skin cells at the wound edge grow actin filaments at their leading edge and crawl across the gap to close it.
Actin growth at the front edge is the engine of cell crawling.
Actin is one of the most abundant and conserved proteins in nature, but the same protein has very different roles depending on what accessory proteins bind it — actin alone does not decide whether it makes a stiff bundle or a loose web.