Transmembrane proteins

Transmembrane proteins form an important class of membrane proteins that permeate the entire phospholipid bilayer of a cell. They account for almost one third of the total protein content of cells and are of great importance for the functionality of the cell and the cell organelles.

Transmembrane proteins have lipophilic and hydrophilic regions. The lipophilic sections of the transmembrane proteins interact with the lipid molecules inside the bilayer. The hydrophilic sections (= tunnel proteins) protrude from the lipid bilayer on both sides. Transmembrane proteins are mostly glycosylated. Their oligosaccharide chains are always located on the extracellular side of the membrane. Other proteins associated with membranes are bound to only one of the two outer sides of the membrane

Transmembrane proteins perform their function as cell adhesion molecules, receptors, ion channels or structural proteins. They are specific to each membrane type of a cell. The endoplasmic reticulum has different membrane proteins than, for example, the plasma membrane or the mitochondria. This reflects the different functions of different biomembranes. A change in the function of the biomembrane is associated with the exchange of membrane proteins.

The part of the protein that crosses the membrane is called the transmembrane domain.

The membrane in question can be either the plasma membrane or the membrane of an organelle in the cytosol. Transmembrane proteins include ion channels, transmembrane receptors, transporters, ATP-dependent pumps, cell adhesion molecules and others. A distinction is made between single-pass transmembrane proteins, which cross the membrane only once, and multi-pass transmembrane proteins, which cross the membrane several times.

• The single-pass transmembrane proteins are referred to as type I transmembrane proteins if the N-terminus is extracellular.
• The single-pass transmembrane proteins are called type II transmembrane proteins if the N-terminus is intracellular.
• Type III membrane proteins contain several transmembrane helices (so-called multipass proteins)
• Type IV membrane proteins consist of several type I and type II proteins

• Type V and VI membrane proteins contain a lipid anchor that is covalently linked to the amino acid chain and anchors the protein in the membrane.
  

• Beta-barrel membrane proteins, which span the membrane with antiparallel folded-barrel structures.

The transmembrane proteins include:

• Ion channels
• transmembrane receptors
• transporters
• ATP-dependent pumps
• Cell adhesion molecules etc.