Actin Filaments: Function & Structure Video

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  • 0:00 Cytoskeleton & Actin Filaments
  • 0:35 Actin Filament Structure
  • 1:40 Actin Crosslinking
  • 2:05 Functions: Muscle Contraction
  • 3:15 Cell Shape & Adhesion
  • 4:20 Lesson Summary
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Lesson Transcript
Instructor: Katy Metzler

Katy teaches biology at the college level and did her Ph.D. work on infectious diseases and immunology.

The cytoskeleton provides structure and shape to cells. In this lesson, learn about actin filaments, a kind of cytoskeletal filament that is important for cell shape, muscle contraction, and cell adhesion.

Cytoskeleton & Actin Filaments

Without our skeleton, we would just be a big sloppy lump of organs, muscles and skin. Our skeleton gives us our shape, and with it the structure required to move around and do things. The same thing is true for cells. Without the cytoskeleton, cells would not be able to maintain and change their shapes as needed, to resist physical stresses, to transport vesicles through the cytosol, or to move around autonomously, just to name a few. The cytoskeleton is clearly a very important part of the cell. Here, we will learn about one of type of cytoskeletal filament, actin filaments, and some of their functions in cells.

Actin Filament Structure

Actin filaments are the smallest cytoskeletal filaments, with a diameter of 7 nm. They are thin, relatively flexible threads that can be crosslinked together in different ways to form very different structures.

Actin monomers are called globular actin or G-actin. As their name suggests, they are fairly globe-shaped in structure. At the right concentration of monomers, they can polymerize head to tail to form filamentous actin or F-actin. F-actin threads associate with each other in a thin double-helical structure, as shown in this diagram.

G-actin monomers polymerize into F-actin filaments.
Diagram of a basic actin filament.

Because the G-actin monomers are arranged in the same orientation, actin filaments have two distinct ends. The ends are called plus (+) and minus (-). The plus end grows about 5-10 times faster than the minus end. The plus and minus ends are also important because motor proteins such as myosin move along the actin filament only in one direction. This is important in muscle contraction.

Actin Crosslinking

There are many proteins in the cell that can link actin filaments to each other in various three-dimensional structures. Some, like alpha actinin, villin and fimbrin, link individual filaments together in actin bundles where the filaments are all lined up parallel to one another. Others, like spectrin and filamin, cross-link actin filaments at angles to each other, forming actin networks, which are web or cushion-like structures. In addition, actin bundles and actin networks change the cell's shape and structure in different ways.

Functions of Actin: Muscle Contraction

Actin filaments have many functions within the cell. For example, our muscle cells are packed with actin filaments arranged in bundles by alpha actinin. As you can see in the diagram, the motor protein myosin is located in between the parallel actin filaments. By 'walking' toward the plus ends of the actin filaments, myosin slides the filaments inwards so that the whole structure gets shorter. This is what makes our muscles contract.

A diagram of how muscle contraction works. The actin bundle contracts as the motor protein myosin moves towards the plus ends of the filaments.
A diagram of how muscle contraction works.

Villin and fimbrin assemble actin filaments into tight, dense bundles that poke out of the cell surface to make microvilli.

A single microvillus formed by a dense bundle of actin under the plasma membrane.
A diagram of a microvillus formed by an actin bundle.

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