A urea transporter is a

membrane transport protein A membrane transport protein is a membrane protein involved in the movement of ions, small molecules, and macromolecules, such as another protein, across a biological membrane. Transport proteins are integral membrane proteins, integral transmembr ...

, transporting

urea Urea, also called carbamide (because it is a diamide of carbonic acid), is an organic compound with chemical formula . This amide has two Amine, amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest am ...

. Humans and other mammals have two types of urea transport proteins, UT-A and UT-B. The UT-A proteins are important for renal urea handling and are produced by alternative splicing of the SLC14A2 gene. Urea transport in the kidney is regulated by vasopressin. The structure of a urea transport family protein from '' Desulfovibrio vulgaris'' was determined by

x-ray crystallography X-ray crystallography is the experimental science of determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to Diffraction, diffract in specific directions. By measuring th ...

. The structure has a pathway through the membrane that is similar to that of

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 (electrophysiol ...

proteins, accounting for the ability of urea transport proteins to move up to one million urea molecules per second across the membrane. Urea transporters can be inhibited by the action of urea analogues like thiourea and glycosides like phloretin. Their inhibition results in increased diuresis due to urea induced osmosis in the collecting ducts of the

kidney In humans, the kidneys are two reddish-brown bean-shaped blood-filtering organ (anatomy), organs that are a multilobar, multipapillary form of mammalian kidneys, usually without signs of external lobulation. They are located on the left and rig ...

Types

In mammals, there are two urea transporter

gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

s: UT-A ('' SLC14A2'') and UT-B ('' SLC14A1''). Multiple protein isoforms derived from each gene are produced by

alternative splicing Alternative splicing, alternative RNA splicing, or differential splicing, is an alternative RNA splicing, splicing process during gene expression that allows a single gene to produce different splice variants. For example, some exons of a gene ma ...

and alternative promoters.

UT-A1

Urea transporter A1 transports urea across the apical membrane into the intracellular space of luminal cells in the

inner medullary collecting duct The collecting duct system of the kidney consists of a series of tubules and ducts that physically connect nephrons to a minor calyx or directly to the renal pelvis. The collecting duct participates in electrolyte and fluid balance through rea ...

of the

kidneys In humans, the kidneys are two reddish-brown bean-shaped blood-filtering organs that are a multilobar, multipapillary form of mammalian kidneys, usually without signs of external lobulation. They are located on the left and right in the retro ...

. UT-1 is activated by ADH, but is a passive transporter. It reabsorbs up to 70% of the original filtered load of urea.

UT-A2

Urea Transporter 2 transports urea across the apical membrane into the luminal space of cells in the thin descending loop of Henle of the

UT-A3

Urea transporter 3 transports urea into the interstitium of the Inner Medullary Collecting Duct.

UT-A4

Urea transporter 4 has been detected in rat but not mouse kidney medulla.

UT-A5

Urea transporter 5 is not expressed in the kidney but in the testis.

UT-B

UT-B is widely expressed and has been studied in erythrocytes,

, intestine, and at the

blood–brain barrier The blood–brain barrier (BBB) is a highly selective semipermeable membrane, semipermeable border of endothelium, endothelial cells that regulates the transfer of solutes and chemicals between the circulatory system and the central nervous system ...

. The SLC14A1 gene codes for the UT-B protein. UT-B is expressed at the basolateral and apical regions of the descending Vasa recta.

References

{{Membrane transport proteins, bg, bg1 Transmembrane transporters Renal physiology Solute carrier family