page_title: Transmembrane protein
section_title: Classification by topology
hierarchical_section_title: Transmembrane protein / Types / Classification by topology
caption_attribution_description: English: Group I and II transmembrane proteins have opposite orientations. Group I proteins have the N terminus on the far side and C terminus on the cytosolic side. Group II proteins have the C terminus on the far side and N terminus in the cytosol. Adapted from Genes IX by Benjamin Lewin.
context_page_description: A transmembrane protein is a type of integral membrane protein that spans the entirety of the cell membrane. Many transmembrane proteins function as gateways to permit the transport of specific substances across the membrane. They frequently undergo significant conformational changes to move a substance through the membrane. They are usually highly hydrophobic and aggregate and precipitate in water. They require detergents or nonpolar solvents for extraction, although some of them can be also extracted using denaturing agents.
  The peptide sequence that spans the membrane, or the transmembrane segment, is largely hydrophobic and can be visualized using the hydropathy plot. Depending on the number of transmembrane segments, transmembrane proteins can be classified as single-span or multi-span. Some other integral membrane proteins are called monotopic, meaning that they are also permanently attached to the membrane, but do not pass through it.
context_section_description: This classification refers to the position of the protein N- and C-termini on the different sides of the lipid bilayer. Types I, II, III and IV are single-pass molecules. Type I transmembrane proteins are anchored to the lipid membrane with a stop-transfer anchor sequence and have their N-terminal domains targeted to the endoplasmic reticulum (ER) lumen during synthesis (and the extracellular space, if mature forms are located on cell membranes). Type II and III are anchored with a signal-anchor sequence, with type II being targeted to the ER lumen with its C-terminal domain, while type III have their N-terminal domains targeted to the ER lumen. Type IV is subdivided into IV-A, with their N-terminal domains targeted to the cytosol and IV-B, with an N-terminal domain targeted to the lumen. The implications for the division in the four types are especially manifest at the time of translocation and ER-bound translation, when the protein has to be passed through the ER membrane in a direction dependent on the type.
caption: Group I and II transmembrane proteins have opposite final topologies.    Group I proteins have the N terminus on the far side and C terminus on the cytosolic side.    Group II proteins have the C terminus on the far side and N terminus in the cytosol. However final topology not the only criterion for defining transmembrane protein groups, rather location of topogenic determinants and mechanism of assembly is considered in the classification[9]
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