# Organic cation transport protein

An organic cation transport protein mediates the transport of organic cations across the cell membrane. These proteins are members of the solute carrier family, subfamily 22. This family of proteins can also transport zwitterions and anions, though it is a different subfamily of solute carrier proteins than the organic anion transporters.

## Function

Organic cation transport proteins (OCTs) are involved in membrane transport through facilitated diffusion of organic cations and some weak bases. They are crucial for the disposal of many organic cations that come from drugs or other environmental sources. They also allow for the recycling of necessary organic cations. Some of these organic cation transport proteins are compound-specific, and others transport a broad range of cations across the membrane. For example, the disposing of the blood sugar medication Metformin is dependent on organic cation transport proteins$^{[1]}$.

## Structure

The general structure of an OCT or OCTN consists of 11-12 alpha-helical transmembrane protein domains. These create a transmembrane unit that creates loops on both sides of the membrane. There is typically a large extracellular loop with glycosylation sites at the beginning of the transporter, followed by many smaller loops that connect the transmembrane protein domains on the extracellular side. The loops found on the inside of the cell contain many sites for phosphorylation$^{[2]}$. There is also an intracellular loop between the transmembrane domains 6 and 7 that contains phosphorylation sites that are believed to contribute to regulation of activity of the organic cation transport protein. Some of these phosphorylation sites are shared between the different organic cation transport proteins$^{[3]}$.

## Nomenclature

The various types of organic cation transport proteins are often referred to in its respective abbreviated form or by the gene that codes it. For example, OCT1 is coded by the SLC22A1 gene. Similarly, OCT2 is often called SLC22A2.

## Proteins

| Abbreviation | Protein name | Location | Transported solute(s) | Associated disorder(s) |
| SLC22A1 | Solute carrier family 22 member 1 | Liver, brain (cerebellum) | | |
| SLC22A2 | Solute carrier family 22 member 2 | Primarily in kidney, also in brain | | |
| SLC22A3 | Solute carrier family 22 member 3 | Brain and kidney | Histamine, dopamine, serotonin, norepinephrine, MPP$^+$ | |
| SLC22A4 | Solute carrier family 22 member 4 | | Sodium ion and ergothioneine | |
| SLC22A5 | Solute carrier family 22 member 5 | | | Primary carnitine deficiency |
| SLC22A6 | Solute carrier family 22 member 6 | | | |
| SLC22A7 | Solute carrier family 22 member 7 | | | |
| SLC22A8 | Solute carrier family 22 member 8 | | | |
| SLC22A9 | Solute carrier family 22 member 9 | | | |
| SLC22A10 | Solute carrier family 22 member 10 | | | |
| SLC22A11 | Solute carrier family 22 member 11 | | | |
| SLC22A12 | Solute carrier family 22 member 12 | Kidney | Uric acid | Hyperuricemia and hypouricemia |
| SLC22A13 | Solute carrier family 22 member 13 | | | |
| SLC22A14 | Solute carrier family 22 member 14 | | | |
| SLC22A15 | Solute carrier family 22 member 15 | | | |
| SLC22A16 | Solute carrier family 22 member 16 | | | |
| SLC22A17 | Solute carrier family 22 member 17 | | | |
| SLC22A18 | Solute carrier family 22 member 18 | | | Beckwith-Wiedemann syndrome, Wilms tumor, rhabdomyosarcoma, adrenocortical carcinoma, and lung, ovarian, and breast cancer |
| SLC22A19 | Solute carrier family 22 member 19 | | | |
| SLC22A20 | Solute carrier family 22 member 20 | | | |