A transmembrane protein is a protein which spans the entire length of the cell membrane. It is embedded between the phospholipids, providing a channel through which molecules and ions can pass into the cell. Transmembrane proteins also facilitate communication between cells by interacting with chemical messengers. Many biological processes, such as the metabolism of glucose and the production of fatty acids, are triggered after a particular transmembrane protein is activated.
The insulin receptor is an example of a transmembrane protein which interacts with a chemical messenger, namely insulin. The receptor acts as a target on the surface of the cell for the insulin molecule. After the molecule docks with the receptor, the receptor typically releases chemicals that cause the movement of a glucose transporter to the surface of the cell. This allows the cell to absorb large amounts of glucose from the external environment, leading to glucose metabolism and eventually energy production.
Another job of the transmembrane protein is to shuttle ions, such as sodium and potassium, across the cell membrane to maintain the chemical environment. Some cells cannot perform specific tasks if the ion channels are not working properly. An important example of this is the voltage-gated ion channels of nerve cells. In the resting state, the ion channel is typically closed, preventing ions from crossing the membrane. As soon as a stimulus is detected, such as a cut or burn, a nerve impulse is sent from one end of the nerve cell to the other end. This can only occur if the ion channels open and allow the ions to flow through the cell membrane.
In order to organize properly, cells also require transmembrane proteins to examine the environment within which the cell resides. For example, muscle cells generally organize around other muscle cells, while skin cells organize around other skin cells. Integrins are a broad category of transmembrane proteins that perform this organizing function. Integrins also anchor cells to substrates, facilitating cell migration and wound healing. A cell's growth, division and death are generally dependent on the signals received by integrins.
A transmembrane protein can be classified as alpha helical or beta barrel, depending on how the protein chain is organized. Alpha helical proteins consist of a single chain while the beta barrel proteins have several protein chains organized side by side. The alpha helical protein is generally coiled, and the beta barrel protein is twisted into a closed structure that resembles a barrel.