View source for Membrane transport protein



{{SI}}

{{EH}}

==Overview==

A '''membrane transport protein''' (or simply '''transporter''') is a [[protein]] involved in the movement of [[ion]]s, small molecules, or macromolecules, such as another [[protein]] across a [[biological membrane]].  Transport proteins are [[integral membrane protein]]s; that is they exist within and span the membrane across which they transport substances.  The proteins may assist in the movement of substances by facilitated diffusion or active transport. 

==Facilitated diffusion==
A [[facilitated diffusion]] protein speeds the movement of a chemical through a membrane in the absence of [[energy]] input; therefore, the transported chemical can only move down a [[concentration gradient]].  This can be accomplished by the formation of a high-specificity pore or channel that spans the membrane. These polar "holes" through the membrane are lined by specific amino acids residues which lower the energy barrier to the movement of polar molecules.

==Active transport==
Transport proteins are also used in [[active transport]], which by definition does require an energy input. 

Chemiosmotic transport utilizes electrochemical gradients to drive transport.  As the creation and maintenance of chemiosmotic gradients require energy input from the cell, this is a form of active transport.  [[Prokaryote]]s typically use [[hydrogen ion]]s as the driving force for chemiosmotic transport, while [[eukaryote]]s typically use [[sodium]] ions. A [[symporter]]/coporter transports a chemical in the same direction as the electrochemical gradient, while an [[antiporter]] moves the target chemical in a direction opposite to the gradient.  

The [[uniporter]] is also often included as a category of chemiosmotic transporter, although a uniporter can also be considered a [[facilitated diffusion]] protein on the basis of function.

===Binding dependent active transport===
Binding dependent active transport also moves the targeted chemical against a [[concentration gradient]], but uses stored chemical energy, typically in the form of [[adenosine triphosphate]], to power the transport.  Generally speaking, a binding dependent transport system consists of a membrane spanning component with a high degree of specifity.  The membrane spanning component changes configuration with the aid of chemical energy input (often through the use of an associated [[ATPase]] protein), thus translocating the chemical from one side of the membrane to the other.

By some definitions, proteins that [[catalyst|catalyze]] the [[ligation]] of [[phosphate]] or [[coenzyme]] groups to a [[catabolism|catabolized]] chemical can be considered active transport proteins in that they drive the uptake of a chemical by maintaining a steep functional [[concentration gradient]]. This pheonomenon is termed group translocation in the case of sugar phosphorylation and vectoral [[acylation]] or vectoral [[esterification]] in the case of [[fatty acid]] [[coenzyme A]] ligation.

==Classification and examples==
Classification of transmembrane transporters according to [http://www.tcdb.org/ TCDB] and examples of transporters with known 3D structure:

'''1. Channels/Pores'''
**[[Voltage-gated ion channel]] like, including [[potassium channel]]s KcsA and KvAP, and [[inward-rectifier potassium ion channel]] Kirbac [http://opm.phar.umich.edu/families.php?superfamily=8]
**Large-conductance mechanosensitive channel, MscL [http://opm.phar.umich.edu/families.php?superfamily=12]
**[[Mechanosensitive ion channel|Small-conductance mechanosensitive ion channel (MscS)]] [http://opm.phar.umich.edu/families.php?superfamily=11]
**[[Magnesium transporters|CorA metal ion transporters]]  [http://opm.phar.umich.edu/families.php?superfamily=72]
**[[Ligand-gated ion channel]] of [[neurotransmitter]] receptors ([[acetylcholine receptor]]) [http://opm.phar.umich.edu/families.php?superfamily=14]
**[[Aquaporin]]s [http://opm.phar.umich.edu/families.php?superfamily=7]
**[[Chloride channel]]s [http://opm.phar.umich.edu/families.php?superfamily=10]
**Outer membrane auxiliary proteins (polysaccharide transporter) [http://opm.phar.umich.edu/families.php?superfamily=188] 

'''2. Electrochemical Potential-driven transporters'''
**[[Mitochondrial]] [[carrier protein]]s [http://opm.phar.umich.edu/families.php?superfamily=21]
**Major Facilitator Superfamily  (Glycerol-3-hosphate transporter, Lactose [[permease]], and Multidrug transporter EmrD)  [http://opm.phar.umich.edu/families.php?superfamily=15]
**Resistance-nodulation-cell division (multidrug [[efflux]] transporter AcrB, see [[multidrug resistance]])[http://opm.phar.umich.edu/families.php?superfamily=16]
**Dicarboxylate/amino acid:cation symporter (proton glutamate symporter)  [http://opm.phar.umich.edu/families.php?superfamily=20]
**Monovalent cation/proton antiporter (Sodium/proton antiporter 1 NhaA) [http://opm.phar.umich.edu/families.php?superfamily=66]
**[[Neurotransmitter]] sodium symporter [http://opm.phar.umich.edu/families.php?superfamily=67]
**Ammonia transporters [http://opm.phar.umich.edu/families.php?superfamily=13] 
**Drug/Metabolite Transporter  (small multidrug resistance transporter EmrE - the structures  are retracted as erroneous) [http://opm.phar.umich.edu/families.php?superfamily=77]
 
'''3. Primary Active Transporters'''
*Light absorption-driven transporters:
**[[Bacteriorhodopsin]]-like proteins including [[rhodopsin]] (see also [[opsin]])[http://opm.phar.umich.edu/families.php?superfamily=6]
**Bacterial [[photosynthetic reaction centre]]s and [[photosystem]]s I and II [http://opm.phar.umich.edu/families.php?superfamily=2]
**Light harvesting complexes from [[bacteria]] and [[chloroplasts]] [http://opm.phar.umich.edu/families.php?superfamily=1]
*Oxidoreduction-driven transporters
**Transmembrane cytochrome b-like proteins [http://opm.phar.umich.edu/families.php?superfamily=3]: [[coenzyme Q - cytochrome c reductase]] (cytochrome bc1 ); [[cytochrome b6f  complex]]; formate dehydrogenase, respiratory [[nitrate reductase]]; [[succinate - coenzyme Q reductase]] (fumarate reductase); and [[succinate dehydrogenase]].  See [[electron transport chain]].
**[[Cytochrome c oxidase]]s [http://opm.phar.umich.edu/families.php?superfamily=4] from [[bacteria]] and [[mitochondria]]
*Electrochemical potential-driven transporters
**Proton or sodium translocating F-type and V-type [[ATPase]]s [http://opm.phar.umich.edu/families.php?superfamily=5]
*P-P-bond hydrolysis-driven transporters
**P-type [[calcium ATPase]]  (five different conformations)  [http://opm.phar.umich.edu/families.php?superfamily=22]
**Calcium ATPase regulators [[phospholamban]] and sarcolipin[http://opm.phar.umich.edu/families.php?superfamily=70]
**[[ABC transporter]]s: [http://opm.phar.umich.edu/families.php?superfamily=17 BtuCD], [http://opm.phar.umich.edu/families.php?superfamily=18 multidrug transporter], and  [http://opm.phar.umich.edu/families.php?superfamily=198 molybdate uptake transporter]
**General [[secretory pathway]] (Sec) [[translocon]] (preprotein translocase SecY) [http://opm.phar.umich.edu/families.php?superfamily=19]

'''4. Group Translocators'''

'''5. Transport Electron Carriers'''
**Disulfide bond formation protein B (DsbB) [http://opm.phar.umich.edu/protein.php?search=2hi7]
'''8. Accessory Factors Involved in Transport'''

==More examples==

* [[Karyopherin]]
* [[Mitochondrial membrane transport protein]]
* [[Glucose transporter]]
* [[ATP-binding cassette transporter genes]]
** [[P-glycoprotein]]
** [[CD98]]
* [[CD36]]
* [[Neurotransmitter transporter]]s
** [[Glutamate transporter]]
** [[Norepinephrine transporter]]
** [[Vesicular monoamine transporter]]
* [[V-ATPase]]
* [[Ion transporter]]s
** [[Na+/K+-ATPase]]
** [[Plasma membrane Ca2+ ATPase|Plasma membrane Ca<sup>2+</sup> ATPase]]
** [[Proton pump]]
** [[Hydrogen potassium ATPase]]
** [[Sodium-chloride symporter]]

==See also==
* [[Carrier protein]]
* [[Ion channel]]
* [[P-loop]]
* [[Solute carrier family]] (classification)
* [[TC number]] (classification)
* [[Vesicular transport protein]]

==External links==
* {{Dorlands|p_37|12672349}}
* {{MeshName|Membrane+transport+proteins}}

{{Carrier proteins}}
{{Membrane transport proteins}}
{{Ion channels}}
{{Ion pumps}}
{{Neurotransmitter transporters}}
[[Category:Transport proteins|*]]
[[Category:Transmembrane transporters]]

{{SIB}}

{{WH}}
{{WikiDoc Sources}}