Transmembrane protein

Transmembrane protein (TP) is a kind of integral membrane protein, it spans the entirety of cell membrane. Many transmembrane proteins have gateway function to permit specific substance transportation across the membrane. In this process, these proteins frequently undergo significant alternation in conformation, in order to move the substances through the membrane. TP are usually highly hydrophobic, then aggregate and precipitate in water. Therefore, detergents or nonpolar solvents are applied for extraction, and beta-barrels can be extracted by denaturing agents.

According to the number of transmembrane segments, transmembrane proteins are classified into single-span (bitopic) and multi-span (polytopic). There are some other integral membrane proteins, called monotopic proteins. These proteins can attach to the membranes permanently, but never pass through.

Classification of Transmembrane Protein

Structural classification

There are two basic structure of transmembrane proteins: alpha-helical and beta barrels. Alpha-helical proteins are existing in plasma membrane of eukaryotic cells, inner membranes of bacterial cells, or outer membrane of some bacteria. Beta barrel proteins are found only in cell wall of gram-positive bacteria, outer membranes of gram-negative bacteria, outer membranes of mitochondria and chloroplasts.

Topology classification

Topology classification refers to the N- and C-termini position of the protein on different sides of the lipid bi-layer. Type I, II, III and IV are single-pass molecules.

  1. Type I transmembrane proteins have N-terminal domains to the endoplasmic reticulum (ER) lumen in synthesis. This kind of protein also anchor to the lipid membrane with the stop-transfer anchor sequence.
  2. Type II and type III have the signal-anchor sequence, type II target to the ER lumen with C-terminal domain, while type III target with N-terminal domain.
  3. Type IV is divided into IV-A and IV-B, type IV-A have N-terminal domain to target the cytosol, while type IV-B with N-terminal target to the lumen.

Transmembrane protein catalog