Cytoskeletal Proteins: Types & Function

Instructor: Amanda Robb
In this lesson learn about the skeleton of the cell, called the cytoskeleton. Then explore the three main types of proteins that make up the cytoskeleton: microfilaments, intermediate filaments, and microtubules, and the function of each.

What Is the Cytoskeleton?

What holds your body upright? You're probably thinking our skeleton, and you would be correct. Although cells are microscopic, they have a skeletal structure too, the cytoskeleton. The cytoskeleton provides structure like our skeleton, but it also acts like a highway to transport materials around the cell, allows cells to move, and aids in cell division. Just like our skeletal system has different organs, the cytoskeleton has different parts. The parts, made of cytoskeletal proteins, can be divided into three categories: microfilaments, intermediate filaments, and microtubules.

Proteins of the cytoskeleton
cystoskeleton

Microfilaments

Microfilaments are the smallest component of the cytoskeleton. These thin filaments are made of actin proteins strung together in a spiraling chain. They serve several purposes in a cell: support, movement, and cell division.

Support

Actin networks coat the inside of the cell membrane, giving it structure and support. Picture tent poles in a circus tent. The poles hold up the tent and allow it to open. Actin serves a similar purpose underneath the cell membrane. Like tent poles, the actin network is not arranged randomly. The filaments are arranged so they can create corrals that keep membrane proteins where they are supposed to be. Cells are not just blobs floating around inside your body. They all have specific jobs and every part of the cell is in the exact location that is best for its function. Proteins on the membrane need to be in just the right place to interact with the environment and other cells.

Movement

Although drawings or models of cells are what most individuals see, real cells are moving and changing shape all the time. White blood cells roll around in blood vessels and tissues, patrolling for invaders like bacteria. In order to move like this, actin filaments assemble at the leading edge of the cell. The cell membrane anchors the cell to the substrate, like the blood vessel. The actin filaments in the back of the cell disassemble and the cell rolls forward towards the new attachments. Microfilaments allow this to happen in a split second and in every moment of life.

Cell Division

While microtubules are the main players in cell division, microfilaments are important. After the cell grows, replicates DNA, and is ready to split in two, actin proteins form a band around the center of the cell. The band of actin proteins shrinks, cinching off the cell into two. If you've ever made rolls, bakers suggest using a piece of string to cut the dough. Wrap the string around the middle of the dough and tighten it, cutting the rolls into individual parts. The same thing happens with microfilaments during cell division.

Intermediate Filaments

Intermediate filaments are larger than microfilaments and are thicker and stronger. There are several types, each made of a different protein. The main function of intermediate filaments is structure and support in the cell. In fact some parts of the body are made up entirely of intermediate filaments, like hair and nails, made from a protein called keratin.

Intracellular structures are also made of intermediate filaments. Lamin and desmin are two important proteins that give structure to the nucleus, which stores DNA.

Another function of intermediate filaments is connecting tissues. Inside the body there are multiple types of tissue, such as epithelial tissue lining the organs and surrounding muscle. These tissues need to be tightly linked together to prevent them from shearing apart as you move. The tissues have connecting joints in between them, kind of like the big screws that hold structures in your house together. However, these joints would simply tear apart without something to anchor them inside the cell. The joints in the membrane are attached to thick intermediate filaments that prevent them from tearing, just like the screws connect to larger beams that span the house.

Keratin anchors cell junctions inside the cell, shown in yellow
intermediate filament

Microtubules

Microtubules are the largest filaments in the cytoskeleton, made of the protein tubulin. They are dynamic like actin, constantly remodeling in response to the cell's needs. They have two main jobs in the cell: transporting materials around the cell and cell division.

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