Membrane transport isoforms

As shown in Table 11.1, there are various membrane transporters and isoforms. Many of these transporters are expressed in the GI tract. Among them, the peptide transport systems have received the most attention. Di-, tri-peptide trans-... 

GLUT 5 A facilitative glucose transporter isoform present in the small intestine and other tissues that will transport fructose (and glucose to a lesser extent) across the plasma membrane. 

GLUT 2 Glucose transporter isoform 2 a transport protein located on the plasma membrane of liver, pancreas, intestine, and kidney that will allow glucose to cross the membrane depending on the concentration gradient. GLUT 2 is the transporter that enables export of glucose from the liver. 

OATP is a family of membrane transporters that mediate uptake of endogenous substrates and drugs. Although certain OATP isoforms are selectively involved in hepatic uptake of hydrophobic anions, most OATPs are widely expressed in various physiological barriers in the body, including the intestine, kidney, brain (brain capillary endothelial cells and... 

Most members of classes II aud in have beeu ideutified iu geuomewide database searches. The fuuctiou of these newly discovered transporter isoforms is not yet well defined. Several of them (GLUT-6 and 8) contain motifs that lead to their retention inside the cell, and therefore this prevents their involvement in glucose transport at the plasma membrane. Whetho" mechanisms exist to invoke ceU surface insafion of these transporta is unknown, but it has been established that insulin does not promote translocation of GLUT-6 and GLUT-8 to the ceU surface. 

There are four genes encoding isoforms of the PM Ca ATPase, and alternative splicing generates other isoforms of this single-subunit plasma-membrane transport ATPase. These Ca -transport ATPases are regulated by calmodulin and a variety of kinases and afford cells the ability to regulate cytoplasmic calcium by... 

A second mechanism that impinges on the localization of transporters is through the association with proteins, the most prominent example being syntaxin. Syntaxin is a t-SNARE protein necessary for the fusion of vesicles with the plasma membrane (see the chapter on exocytosis). On the cell surface syntaxin consistently stabilizes the localization of GABA, noradrenaline, glycine, and 5HT transporters the PKCa isoform can sever the interaction with syntaxin suggesting a general mechanism for transporter internalization. 

SNAPs is an acronym for soluble NSF attachment proteins. They were originally discovered as cofactors for NSF that mediate the membrane binding of NSF in in vitro transport assays. Several isoforms of SNAPs exist in mammalian cells. SNAPs are also highly conserved proteins. Crystallographic studies indicated that the proteins form a very stiff and twisted sheet that is formed by a series of antiparallel and tightly packed helices connected by short loops. 

In the family of cation pumps, only the Na,K-ATPase and H,K-ATPase possess a p subunit glycoprotein (Table II), while the Ca-ATPase and H-ATPase only consist of an a subunit with close to 1 000 amino acid residues. It is tempting to propose that the p subunit should be involved in binding and transport of potassium, but the functional domains related to catalysis in Na,K-ATPase seem to be contributed exclusively by the a subunit. The functional role of the P subunit is related to biosynthesis, intracellular transport and cell-cell contacts. The P subunit is required for assembly of the aj8 unit in the endoplasmic reticulum [20]. Association with a j8 subunit is required for maturation of the a subunit and for intracellular transport of the xP unit to the plasma membrane. In the jSl-subunit isoform, three disulphide... 

Biochemical studies of plasma membrane Na /H exchangers have been directed at two major goals (1) identification of amino acids that are involved in the transport mechanism and (2) identification and characterization of the transport pro-tein(s). To date, most studies have been performed on the amiloride-resistant form of Na /H" exchanger that is present in apical or brush border membrane vesicles from mammalian kidney, probably because of the relative abundance of transport activity in this starting material. However, some studies have also been performed on the amiloride-sensitive isoform present in non-epithelial cells. 

It is well known that chemical compo.sition of rhizosphere solution can affect plant growth. Particularly, uptake of nutrients may be considerably influenced by the ionic concentration of the rhizosphere solution (40). Despite the difficulty of defining the exact concentration of ions in the rhizosphere surrounding each root (or even root portion), it has been unequivocally demonstrated that plants have evolved mechanisms to cope with the uneven distribution of ions in the root surrounding in order to provide adequate supply of each essential nutrient (41). These mechanisms include expression of transporter genes in specific root zones or cells and synthesis of enzymes involved in the uptake and assimilation of nutrients (40,43). Interestingly, it has been shown that specific isoforms of the H -ATPase are expressed in the plasma membrane of cell roots it has been proposed that the expression of specific isoforms in specific tissues is relevant to nutrient (nitrate) acquisition (44) and salt tolerance (45). 

The uniquely high resolution structural data available for the SERCAIa Ca2+ pump illuminates the structure of all P-type transporters. Unlike the Na,K pump, the catalytic subunit of the SERCA Ca2+ pumps is active and does not require association with another subunit. However, the cardiac isoform, SERCA-2a, associates with a small membrane protein, phospholamban, that can... 

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