Cell membrane or plasma membrane is the bilipid layer that covers all cells and plays a very crucial role in the proper functioning of cells. The cell membrane has a number of proteins which are responsible for the various activities of the cell membrane. The amount of proteins present in the cell membrane differs with species and functions of the cells. The typical amount of proteins in cell membrane is around 50%
The membrane proteins are inserted into the cell membrane by endoplasmic reticulum (a cell organelle responsible for protein synthesis) and associated vesicles.
Many membrane proteins are fluid in nature. They can diffuse within the plasma membrane. Other membrane proteins are attached and tied in place by the cytoskeleton.
The membrane proteins can be divided as (a) Integral membrane proteins and (b) Peripheral membrane proteins on the basis of their structure.
Let us now discuss in detail about the structure and functions of these proteins/
Integral or transmembrane proteins
These are proteins which span the entire cell membrane. They are composed of two domains – a hydrophilic domain which interacts with the components of cytosol (the fluid present inside all cells) and the hydrophobic domain which spans the membrane and interacts with components outside the cell. These membrane proteins act as ion channels (aiding in transport of ions like sodium, potassium, calcium, hydrogen etc.), proton pumps and as G-protein coupled receptors (aiding in signal transduction caused by hormones and other molecules)
Peripheral proteins are of two types — those which are attached with the integral membrane proteins and those with the peripheral region of the phospholipid bilayer. These proteins have only temporary interactions with the membranes. Most of these proteins are disintegrated and sent to the cytoplasm where they carry out other functions.
Apart from these two classes of membrane proteins there are lipid anchored proteins which are bound covalently to single or multiple molecules of lipid. These proteins are not in direct contact with the membrane but interact through the hydrophobic lipid molecules. G-proteins are examples f this category of membrane proteins.
Functions of membrane proteins
- Cell-cell communication
- Receptors for various factors like enzymes and hormones
- surface recognition
- aiding in communication between the components of cytoplasm and the extracellular matrix
- cell signaling
- transport of hydrophilic products across the membrane
- cytoskeleton contact
- enzymatic activity
- intracellular joining
- attachment of the cell with the extracellular matrix