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Transporters (membrane transport/carrier proteins) are specialized membrane-spanning proteins that assist in the movement of ions, peptides, small molecules, lipids and macromolecules across a biological membrane.
There are two different types of transport; passive and active. Passive transport requires no energy input as transport follows a concentration gradient. It is sub-divided into two types:
In contrast active transport requires energy (usually from ATP hydrolysis) to transport substances into a cell against the concentration gradient. It is also sub-divided into two types:
Membrane transporters can be also divided into three main classes; ABC transporters, P-type ATPases and the solute carrier family (SLC). ABC transporters are primary active transporters, which transport a wide range of substrates mainly to the outside of a cell membrane or organelle. Their substrates include: lipids and sterols, ions and small molecules, drugs and large polypeptides. ABC transporters play a critical role in the development of multi-drug resistance in cancer cells. Overexpression of ABC transporters can result in chemotherapeutics being pumped out of cell faster than they can enter.
The solute carrier family includes transporters that function by secondary active transport and facilitative diffusion. They are located on the cell membrane as well as on the intracellular membrane of organelles. Examples of the solute carrier family include the biogenic amine transporters (NET, DAT and SERT) and the Na+/H+ exchanger. Inhibitors of the SLC family of transporters have proved useful in the treatment of a variety of disorders, including depression (SERT), epilepsy (GABA transporter) and Parkinson's disease (DAT).