We are all familiar with the workings of muscles, and we all know how to walk and move around. But, did you know that some of the cells in your body are moving around too? Cell migration is a critical process for every type of living organism. Cells in the body will often move from place to place to complete their functions. The process of cell migration is important in such things as wound repair, and cell differentiation. Understanding cell migration is also an important part of cancer research. In cancer, cells that normally would not migrate start to move. This is called metastisis, and is the difference between a benign and malignant cancer. Understanding how cells migrate can help find new cures for cancer, and help us better understand the body in general!
So, how do cells move exactly? That is indeed a very complicated question, however it can surely be said that no migration could take place without an interesting set of proteins called Integrins. These proteins are so named because they integrate the function of the cell with the outside world. This exterior region is generaly known as the Extracellular Matrix. The extracellular matrix is composed of a wide variety of different moecules that "fill in the space" between cells. Integrins are transmembrane proteins which bind ligands found in the extracellular matrix. They exist in cells as "heterodimers." This means they are 2 separate interacting polypeptide chains which are not the same. Here is a drawing of what integrins look like:
The shape of integrins are what is known as a beta barrel. This means that the polypeptide chain wraps around in a barrel shape. This allows the integrin to bind more easily to the extracellular matrix.
HOW INTEGRINS WORK
Integrins can be thought of as velcro on the surface of the cell. When the cell is at rest, most of the integrins are inactive, that is to say they are present, but do not bind the ligands present in the extracellular matrix. Here is a representation of how this works:
When the cell decides to move, it turns on integrins in certain places, and turns them off in other places. Where this takes place depends on which direction the cell wants to move! Integrins on the "front" of the moving cell grip tightly to the extracellular matrix, pulling the cell forward. At the same time, the integrins in the back of the cell must "let go." These are taken up in the cell and recycled. The cell moves by "ruffling" it's membrane. This is done by a series of actin fibers, whose function is controlled by the integrins. These fibers cause the cell membrane to move in certain directions, and the integrins attach to the matrix as this happens, pulling the cell along a micrometer at a time! Here is a depiction of the complex interaction between integrins and actin fibers: