Membrane Proteins

Membrane proteins fall into two major categories: peripheral and integral.

Peripheral membrane proteins associate with phospholipid heads or hydrophilic domains of integral proteins by non-covalent interactions.

In contrast, integral proteins are amphipathic molecules with their hydrophilic region facing the cytoplasm or extracellular fluid and the hydrophobic domain interacting with the phospholipid tails.

Integral membrane proteins can be further classified as monotopic, bitopic, and polytopic proteins.

Monotopic proteins are embedded into a single face of the membrane, whereas bitopic proteins with an alpha-helix structure span the entire lipid bilayer only once.

Polytopic proteins extend across the membrane multiple times and consist of multiple alpha-helices or a cylindrical beta-sheet, known as a beta-barrel. Bitopic and polytopic proteins are also known as transmembrane proteins.

Integral proteins perform diverse functions such as transferring molecules and signals across the cell membrane.

Many peripheral proteins also participate in cell signaling cascades as they can easily detach from the membrane. Other peripheral proteins link the membrane with the cytoskeleton, providing structural support.