A membrane transport protein (or simply transporter) is a
membrane protein involved in the movement of
ions, small
molecule
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bio ...
s, and
macromolecule
A macromolecule is a very large molecule important to biophysical processes, such as a protein or nucleic acid. It is composed of thousands of covalently bonded atoms. Many macromolecules are polymers of smaller molecules called monomers. ...
s, such as another
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respon ...
, across a
biological membrane. Transport proteins are
integral
In mathematics, an integral assigns numbers to functions in a way that describes displacement, area, volume, and other concepts that arise by combining infinitesimal data. The process of finding integrals is called integration. Along with ...
transmembrane protein
A transmembrane protein (TP) 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 frequentl ...
s; that is they exist permanently 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
In cellular biology, ''active transport'' is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration—against the concentration gradient. Active transport requires cellu ...
. The two main types of proteins involved in such transport are broadly categorized as either ''channels'' or ''carriers''. The
solute carriers and
atypical SLCs are secondary active or facilitative transporters in humans.
Collectively membrane transporters and channels are known as the transportome.
Transportomes govern cellular influx and efflux of not only ions and nutrients but drugs as well.
Difference between channels and carriers
A carrier is not open simultaneously to both the extracellular and intracellular environments. Either its inner gate is open, or outer gate is open. In contrast, a channel can be open to both environments at the same time, allowing the molecules to diffuse without interruption. Carriers have binding sites, but pores and channels do not. When a channel is opened, millions of ions can pass through the membrane per second, but only 100 to 1000 molecules typically pass through a carrier molecule in the same time. Each carrier protein is designed to recognize only one substance or one group of very similar substances. Research has correlated defects in specific carrier proteins with specific diseases.
Active transport
Active transport
In cellular biology, ''active transport'' is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration—against the concentration gradient. Active transport requires cellu ...
is the movement of a substance across a membrane against its concentration gradient. This is usually to accumulate high concentrations of molecules that a cell needs, such as glucose or amino acids. If the process uses chemical energy, such as adenosine triphosphate (ATP), it is called
primary active transport
In cellular biology, ''active transport'' is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration—against the concentration gradient. Active transport requires cellul ...
.
Secondary active transport involves the use of an
electrochemical gradient, and does not use energy produced in the cell.
[Ashley, Ruth. Hann, Gary. Han, Seong S. Cell Biology. New Age International Publishers. . p. 113.] Unlike channel proteins which only transport substances through membranes passively, carrier proteins can transport ions and molecules either passively through facilitated diffusion, or via secondary active transport. A carrier protein is required to move particles from areas of low concentration to areas of high concentration. These carrier proteins have receptors that bind to a specific molecule (substrate) needing transport. The molecule or ion to be transported (the substrate) must first bind at a binding site at the carrier molecule, with a certain binding affinity. Following binding, and while the binding site is facing the same way, the carrier will capture or occlude (take in and retain) the
substrate
Substrate may refer to:
Physical layers
*Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached
** Substrate (locomotion), the surface over which an organism lo ...
within its molecular structure and cause an internal translocation so that the opening in the protein now faces the other side of the plasma membrane. The carrier protein substrate is released at that site, according to its binding affinity there.
Facilitated diffusion
Facilitated diffusion is the passage of molecules or ions across a biological membrane through specific transport proteins and requires no energy input. Facilitated diffusion is used especially in the case of large polar molecules and charged ions; once such ions are dissolved in water they cannot diffuse freely across cell membranes due to the hydrophobic nature of the fatty acid tails of the phospholipids that make up the bilayers.
The type of carrier proteins used in facilitated diffusion is slightly different from those used in active transport. They are still transmembrane carrier proteins, but these are gated transmembrane channels, meaning they do not internally translocate, nor require ATP to function. The substrate is taken in one side of the gated carrier, and without using ATP the substrate is released into the cell.
Reverse diffusion
Reverse transport, or ''transporter reversal'', is a phenomenon in which the substrates of a membrane transport protein are moved in the opposite direction to that of their typical movement by the transporter.
Transporter reversal typically occurs when a membrane transport protein is
phosphorylated by a particular
protein kinase
A protein kinase is a kinase which selectively modifies other proteins by covalently adding phosphates to them (phosphorylation) as opposed to kinases which modify lipids, carbohydrates, or other molecules. Phosphorylation usually results in a fu ...
, which is an
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different molecule ...
that adds a
phosphate
In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid .
The phosphate or orthophosphate ion is derived from phosph ...
group to proteins.
Types
(Grouped by
Transporter Classification database categories)
1: Channels/pores
* α-helical protein channels such as
voltage-gated ion channel
Voltage-gated ion channels are a class of transmembrane proteins that form ion channels that are activated by changes in the electrical membrane potential near the channel. The membrane potential alters the conformation of the channel proteins, ...
(VIC),
ligand-gated ion channels(LGICs)
* β-barrel porins such as
aquaporin
* channel-forming toxins, including
colicins,
diphtheria toxin, and others
* Nonribosomally synthesized channels such as
gramicidin
*
Holins; which function in export of enzymes that digest bacterial cell walls in an early step of cell lysis.
Facilitated diffusion occurs in and out of the cell membrane via channels/pores and carriers/porters.
Note:
* Channels:
Channels are either in open state or closed state. When a channel is opened with a slight conformational switch, it is open to both environment simultaneously (extracellular and intracellular)
*
Pores:
Pores are continuously open to these both environment, because they do not undergo conformational changes. They are always open and active.
2: Electrochemical potential-driven transporters
Also named carrier proteins or secondary carriers.
* 2.A: Porters (
uniporters,
symporters
A symporter is an integral membrane protein that is involved in the transport of two (or more) different molecules across the cell membrane in the same direction. The symporter works in the plasma membrane and molecules are transported across the ...
,
antiporters),
SLCs.
**
Excitatory amino acid transporters (EAATs)
***
EAAT1
***
EAAT2
***
EAAT3
***
EAAT4
***
EAAT5
**
Glucose transporter
**
Monoamine transporters, including:
***
Dopamine transporter (DAT)
***
Norepinephrine transporter (NET)
***
Serotonin transporter (SERT)
***
Vesicular monoamine transporters (VMAT)
**
Adenine nucleotide translocator (ANT)
* 2.B: Nonribosomally synthesized porters, such as:
** The
Nigericin family
** The
Ionomycin family
* 2.C: Ion-gradient-driven energizers
3: Membrane transport protein
* 3.A: P-P-bond-hydrolysis-driven transporters :
**
ATP-binding cassette transporter (ABC transporter), such as
MDR MDR may refer to:
Biology
* MDR1, an ATP-dependent cellular efflux pump affording multiple drug resistance
* Mammalian Diving reflex
* Medical device reporting
* Multiple drug resistance, when a microorganism has become resistant to multiple drugs ...
,
CFTR
**
V-type ATPase ; ( "V" related to vacuolar ).
**
P-type ATPase ; ( "P" related to phosphorylation), such as :
***
Na+/K+-ATPase
***
Plasma membrane Ca2+ ATPase
***
Proton pump
**
F-type ATPase; ("F" related to factor), including: mitochondrial
ATP synthase
ATP synthase is a protein that catalyzes the formation of the energy storage molecule adenosine triphosphate (ATP) using adenosine diphosphate (ADP) and inorganic phosphate (Pi). It is classified under ligases as it changes ADP by the formation ...
, chloroplast ATP synthase1
* 3.B: Decarboxylation-driven transporters
* 3.C: Methyltransfer-driven transporters
* 3.D: Oxidoreduction-driven transporters
* 3.E: Light absorption-driven transporters, such as
rhodopsin
Rhodopsin, also known as visual purple, is a protein encoded by the RHO gene and a G-protein-coupled receptor (GPCR). It is the opsin of the rod cells in the retina and a light-sensitive receptor protein that triggers visual phototransduct ...
4: Group translocators
The group translocators provide a special mechanism for the phosphorylation of sugars as they are transported into bacteria (PEP group translocation)
5: Electron carriers
The transmembrane electron transfer carriers in the membrane include two-electron carriers, such as the disulfide bond oxidoreductases (DsbB and DsbD in E. coli) as well as one-electron carriers such as NADPH oxidase. Often these redox proteins are not considered transport proteins.
Examples
Every carrier protein, especially within the same cell membrane, is specific to one type or family of molecules. For example,
GLUT1 is a named carrier protein found in almost all animal cell membranes that transports glucose across the bilayer. Other specific carrier proteins also help the body function in important ways. Cytochromes operate in the
electron transport chain
An electron transport chain (ETC) is a series of protein complexes and other molecules that transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couple ...
as carrier proteins for electrons.
Pathology
A number of inherited diseases involve defects in carrier proteins in a particular substance or group of cells. Cysteinuria (cysteine in the urine and the bladder) is such a disease involving defective cysteine carrier proteins in the kidney cell membranes. This transport system normally removes cysteine from the fluid destined to become urine and returns this essential amino acid to the blood. When this carrier malfunctions, large quantities of cysteine remain in the urine, where it is relatively insoluble and tends to precipitate. This is one cause of urinary stones. Some vitamin carrier proteins have been shown to be overexpressed in patients with malignant disease. For example, levels of
riboflavin carrier protein Riboflavin carrier proteins (RFCPs) together with human serum albumin transport flavin mononucleotide (FMN) in the blood circuit. RFCPs are important in pregnancy.
Studies from India have identified a riboflavin carrier protein (RCP) present in bi ...
(RCP) have been shown to be significantly elevated in people with
breast cancer
Breast cancer is cancer that develops from breast tissue. Signs of breast cancer may include a lump in the breast, a change in breast shape, dimpling of the skin, milk rejection, fluid coming from the nipple, a newly inverted nipple, or ...
.
[Rao, PN, Levine, E et al. Elevation of Serum Riboflavin Carrier Protein in Breast Cancer. Cancer Epidemiol Biomarkers Prev. Volume 8 No 11. pp. 985–990]
See also
*
Cotransport
*
Cotransporter
*
C14orf102
Chromosome 14 open reading frame 102 (accession: AK294958; alias: C14orf102, FLJ14051, FLJ10008, FLJ52106) is a 3810bp protein-encoding gene that is highly conserved among its non-distant orthologs. It contains 20 introns and 8 different RNAs ...
, a 3810bp protein-encoding gene
*
Ion channel
Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore. Their functions include establishing a resting membrane potential, shaping action potentials and other electrical signals by gating the flow of ...
*
Permease
*
P-loop
*
Solute carrier family (classification)
*
TC number (classification)
*
Translocase
*
Vesicular transport protein
A vesicular transport protein, or vesicular transporter, is a membrane protein that regulates or facilitates the movement of specific molecules across a vesicle's membrane. As a result, vesicular transporters govern the concentration of molecules ...
*
Endocytosis
Endocytosis is a cellular process in which substances are brought into the cell. The material to be internalized is surrounded by an area of cell membrane, which then buds off inside the cell to form a vesicle containing the ingested material. E ...
References
Anderle, P., Barbacioru,C., Bussey, K., Dai, Z., Huang, Y., Papp, A., Reinhold, W., Sadee, W., Shankavaram, U., & Weinstein, J. (2004). Membrane Transporters and Channels: Role of the Transportome in Cancer Chemosensitivity and Chemoresistance. Cancer Research, 54, 4294-4301.
External links
*
{{Ion pumps
*
Transmembrane transporters