The two distinct ends. The ends are called

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.

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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.

Microvilli increase the surface area of cells by making lots of tiny hills and valleys.

Our intestinal epithelial cell surfaces are full of microvilli, which is good because our intestines need to absorb as much water as possible to avoid dehydration. The greater the surface area, the more water can be absorbed.In contrast, filamin arranges actin filaments at almost perpendicular angles to each other, creating a very loose meshwork.

A close-up of an actin/filamin meshwork. The filamin molecule is quite flexible and crosslinks actin filaments at approximately perpendicular angles.

A diagram of filamin crosslinking actin filaments.

Cell Shape ; Adhesion

Cells can construct these actin/filamin meshworks right under the plasma membrane to make lamellipodia.

Lamellipodia are membrane projections that help cells crawl across flat surfaces. You can imagine lamellipodia by thinking about what it would look like if you were inside a burlap bag and trying to crawl across the floor. The parts poking around the bag’s surface where your bent arms and legs are trying to reach out would look a little bit like lamellipodia.Our skin and the linings of our organs are made of epithelial cells organized in sheets.

The sheets are made by tight attachment of individual epithelial cells to each other. Actin bundles at adherens junctions help strengthen these epithelial sheets. Neighboring cells are attached to each other’s sides using transmembrane proteins that are linked to actin bundles. The actin bundles form an internal belt around each cell, helping an epithelial sheet withstand side-to-side stretching or pulling forces without the cells separating from each other.

It’s almost like the cells are all holding hands.

Lesson Summary

In summary, we have learned that actin filaments (also called F-actin) are formed by linking together G-actin molecules in a polymer chain. Each actin filament has a plus and a minus end, and motor proteins like myosin ‘walk’ directionally along the filaments. Actin filaments can be arranged in bundles or networks using various kinds of crosslinking proteins.

The bundles and networks are important for muscle contraction, cell shape, and cell adhesion.

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