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An integral, or intrinsic, membrane protein (IMP) [1] is a type of membrane protein that is permanently attached to the biological membrane. All transmembrane proteins can be classified as IMPs, but not all IMPs are transmembrane proteins. [2] IMPs comprise a significant fraction of the proteins encoded in an organism's genome. [3]
In 2008, 150 unique structures of membrane proteins were available, [14] and by 2019 only 50 human membrane proteins had had their structures elucidated. [13] In contrast, approximately 25% of all proteins are membrane proteins. [15] Their hydrophobic surfaces make structural and especially functional characterization difficult.
Schematic representation of transmembrane proteins: 1) a single-pass membrane protein 2) a multipass membrane protein (α-helix) 3) a multipass membrane protein β-sheet. The membrane is represented in light yellow. A transmembrane protein is a type of integral membrane protein that spans the entirety of the cell membrane.
Intramembrane proteases are integral membrane proteins that are polytopic transmembrane proteins with multiple transmembrane helices. [5] [17] Their active sites are located within the transmembrane helices and form an aqueous environment within the hydrophobic lipid bilayer.
An example of a chloride-bicarbonate antiporter is the chloride anion exchanger, also known as down-regulated in adenoma (protein DRA). It is found in the intestinal mucosa, especially in the columnar epithelium and goblet cells of the apical surface of the membrane, where it carries out the function of chloride and bicarbonate exchange. [39]
Integral proteins hold strong association with the lipid bilayer and cannot easily become detached. [9] They will dissociate only with chemical treatment that breaks the membrane. Peripheral proteins are unlike integral proteins in that they hold weak interactions with the surface of the bilayer and can easily become dissociated from the ...
Aquaporins have six membrane-spanning alpha helical domains with both carboxylic and amino terminals on the cytoplasmic side. Two hydrophobic loops contain conserved asparagine–proline–alanine ("NPA motif") which form a barrel surrounding a central pore-like region that contains additional protein density. [3]
As is typical of integral membrane proteins, SLCs contain a number of hydrophobic transmembrane alpha helices connected to each other by hydrophilic intra- and extra-cellular loops. Depending on the SLC, these transporters are functional as either monomers or obligate homo- or hetero-oligomers.