Superfamily Transporters

In this case, the sequence suggests a 14-TM-helix protein with multiple glycosylation and phosphorylation sites. Although differing in molecular detail, it is likely that all members of the equilibrative and Na -linked families will be similar in overall three-dimensional structure and transport mechanism. However, there is a wealth of detailed variation upon which selectivity in drug transport or transporter inhibition may eventually be based. 

Cells differ in their reliance on nucleoside uptake and salvage versus de novo biosynthetic pathways for normal growth, and, hence, they differ in their sensitivity to nucleoside drugs. Table 14.5 [adapted from Tables 1-4 in Cass (61)] lists some nucleoside drugs, diseases for which they have been used, and the transporters that recognize them. In addition to the es, ei, and N1-N5 nucleoside transporters, some nucleoside drugs also utilize nucleobase (NB) transporters. 

Nucleoside drug Clinical indication Transporter specificity  

Adapted from Tables 1-4 in Cass CE. Nucleoside transport. In Georgopapadakou NH, ed. Drug transport in antimicrobial and anticancer chemotherapy. New York Marcel Dekker, Inc. 1995. p. 408-51. 

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Hinoshita, E., Uchiumi, T., Taguchi, K., Kinukawa, N., Tsuneyoshi, M., Maehara, Y., Sugimachi, K. and Kuwano, M. (2000) Increased expression of an ATP-binding cassette superfamily transporter, multidrug resistance protein 2, in human colorectal carcinomas. Clinical Cancer Research, 6, 2401-2407. 

The ABC-transporter superfamily represents a large group of transmembrane proteins. Members of this family are mainly involved in ATP-dependent transport processes across cellular membranes. These proteins are of special interest from a pharmacological point of... 

Currently, five different molecular classes of mdr efflux pumps are known [5], While pumps of the the ATP-binding cassette (ABC) transporter superfamily are driven by ATP hydrolysis, the other four superfamilies called resistance-nodulation-division (RND), major facilitator superfamily (MFS), multidrug and toxic compound extrusion (MATE), and small multidrag resistance transporter (SMR) are driven by the proton-motive force across the cytoplasmic membrane. Usually a single pump protein is located within the cytoplasmic membrane. However, the RND-type pumps which are restricted to Gram-negative bacteria consist of two additional components, a periplasmic membrane fusion protein (MFP) which connects the efflux pump to an outer... 

An alternative to most of these mechanisms is the existence of efficient efflux systems, so that toxic concentrations of the drug are not achieved. There are three major families of proton-dependent multidrug efflux systems (1) the major facilitator superfamily, (2) the small multidrug resistance family, and (3) the resistance/nodulation/cell division family (Paulsen et al. 1996). It should be emphasized that several of these systems are involved not with antibiotic efflux but with, for example, acriflavine, chlorhexidine, and crystal violet. An attempt is made only to outline a few salient features of the resistance/nodulation/cell division family that mediates antibiotic efflux, and these are given in Table 3.3 (Nikaido 1996). They consist of a transporter, a linker, and an outer membrane channel. 

The determination of the structure of the iron transporter, ferric-binding, protein (hFBP)t from Haemophilus influenzae (Bruns et ah, 1997) at 0.16 nm resolution shows that it is a member of the transferrin superfamily, which includes both the transferrins and a number of periplasmic binding proteins (PBP). The PBPs transport a wide variety of nutrients, including sugars, amino acids and ions, across the periplasm from the outer to the inner (plasma) membrane in bacteria (see Chapter 3). Iron binding by transferrins (see below) requires concomitant binding of a carbonate anion, which is located at the N-terminus of a helix. This corresponds to the site at which the anions are specifically bound in the bacterial periplasmic sulfate- and... 

Figure 5.3 The deduced evolutionary tree for selected members of the transferrin superfamily, based on comparisons of structures and sequences. The tree combines the transferrins with a number of prokaryotic periplasmic transport proteins. From Bruns et al., 1997. Reproduced by permission of Nature Publishing Group.

Shepard, I. L. Jenkins, D. C. Duckworth, J. R. Sportsman et al. Association of intestinal peptide transport with a protein related to the cadherin superfamily, Science 1994, 264, 430-433... 

Fei, Y. J., V. Ganapathy, and F. H. Leibach. Molecular and structural features of the proton-coupled oligopeptide transporter superfamily. Prog. Nucleic Acid. Res. Mol. Biol. 1998, 58, 239-261. 

Geourjon, C., et al. A common mechanism for ATP hydrolysis in ABC transporter and helicase superfamilies. Trends Biochem. Sci. 2001,... 

The presence at the BBB of members of the multidrug resistance-associated protein (MRPs) family, whose members preferentially transport anionic compounds, is still controversial. The seven members of the MRP family belong, like P-gp, to the ATP-binding cassette (ABC) protein superfamily. Mrpl has been found at the BBB in isolated rat brain capillaries, primary cultures of brain capillary endothelial cells and in immortalized capillary endothelial cells, but not in human brain capillaries [59]. Another member, MRP2 has been found at the luminal membrane of the brain endothelial cells [60]. However, further studies are required to show that there are MRP transporters at the BBB (Figure 15.5). As for P-gp, a functional Mrpl was found in primary cultured rat astrocytes [56] and it has been shown to take part in the release of glutathione disulfide from brain astrocytes under oxidative stress [61]. 

There are also monomeric G-proteins. Just like the trimeric G-pro-teins, they are involved as signal relays and timers. The Ras superfamily relays signals from receptor tyrosine kinases to downstream elements that eventually regulate transcription. Rho and Rac relay signals from cell-surface receptors to the cytoskeleton, while Rab regulates intracellular transport of vesicles. Regardless of what they do, they use the timer mechanism provided by the G-protein. Three-letter acronyms (TLA), such as Ras, Rho, and Rab, are difficult to remember, sometimes even when you know what the letters stand for. Unfortunately, there s nothing you can do about this except to memorize them. 

The primary transporters discussed in this chapter belong to three distinct genomic superfamilies that differ markedly in structure and reaction mechanism 74... 

P-type transporters share the same general reaction mechanism. Most pumps belonging to the P-type transporter superfamily have evolved to create cation gradients. 

The ABC transporters are products of one of the largest gene superfamilies. Each consists of two cytoplasmic nucleotide-binding domains (NBDs) and two transmembrane domains (TMDs). The NBDs are highly conserved across the ABC family and contain motifs typical of ATP-binding sites, whereas the TMD structures vary, probably because they are adapted to the wide variety of substrates. In eukaryotes the C-terminal of each NBD is linked to a TMD. In some cases the functional unit is (NBD-TMD)2 and, in others, the first TMD is covalently linked to the second NBD. 

SLC2 (solute carrier) superfamily consists of 12 glucose transporters (GLUT1-12) and one H+-myo-inositol cotransporter (HMIT or GLUT13). They all have 12 transmembrane segments with the N- and C-termini both on the cytoplasmic aspect and a specific N-linked oligosaccharide side-chain on either the first or fourth extracellular loop. 

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