Membrane protein

Source: Wikipedia, the free encyclopedia.
Membrane protein complexes of photosynthesis in the thylakoid membrane

Membrane proteins are common proteins that are part of, or interact with, biological membranes. Membrane proteins fall into several broad categories depending on their location. Integral membrane proteins are a permanent part of a cell membrane and can either penetrate the membrane (transmembrane) or associate with one or the other side of a membrane (integral monotopic). Peripheral membrane proteins are transiently associated with the cell membrane.

Membrane proteins are common, and medically important—about a third of all human proteins are membrane proteins, and these are targets for more than half of all drugs.[1] Nonetheless, compared to other classes of proteins, determining membrane protein structures remains a challenge in large part due to the difficulty in establishing experimental conditions that can preserve the correct conformation of the protein in isolation from its native environment.

Function

Membrane proteins perform a variety of functions vital to the survival of organisms:[2]

The localization of proteins in membranes can be predicted reliably using

hydrophobicity analyses of protein sequences, i.e. the localization of hydrophobic amino acid
sequences.

Integral membrane proteins

β-sheet
protein
The membrane is represented in light-brown.

nonpolar solvents, or sometimes denaturing agents.[citation needed] They can be classified according to their relationship with the bilayer
:

Peripheral membrane proteins

calcium ion
)

electrostatic, and other non-covalent interactions. Peripheral proteins dissociate following treatment with a polar reagent, such as a solution with an elevated pH or high salt concentrations.[citation needed
]

Integral and peripheral proteins may be post-translationally modified, with added

GPI
(glycosylphosphatidylinositol), which may be anchored in the lipid bilayer.

Polypeptide toxins

irreversibly with the lipid bilayer and become reversibly or irreversibly membrane-associated.[citation needed
]

In genomes

Membrane proteins, like soluble

disordered proteins, are common.[9] It is estimated that 20–30% of all genes in most genomes encode for membrane proteins.[10][11] For instance, about 1000 of the ~4200 proteins of E. coli are thought to be membrane proteins, 600 of which have been experimentally verified to be membrane resident.[12] In humans, current thinking suggests that fully 30% of the genome encodes membrane proteins.[13]

In disease

Membrane proteins are the

medicinal drugs.[1] Among the human diseases in which membrane proteins have been implicated are heart disease, Alzheimer's and cystic fibrosis.[13]

Purification of membrane proteins

Although membrane proteins play an important role in all organisms, their purification has historically, and continues to be, a huge challenge for protein scientists. In 2008, 150 unique structures of membrane proteins were available,

water-soluble, but these can also alter protein structure and function.[13] Making membrane proteins water-soluble can also be achieved through engineering the protein sequence, replacing selected hydrophobic amino acids with hydrophilic ones, taking great care to maintain secondary structure while revising overall charge.[13]

polyhistidine-tag is a commonly used tag for membrane protein purification,[17] and the alternative rho1D4 tag has also been successfully used.[18][19]

See also

References

Further reading

External links

Organizations

Membrane protein databases