François Verrey

CATs and HATs: the SLC7 family of amino acid transporters

The SLC7 family is divided into two subgroups, the cationic amino acid transporters (the CAT family, SLC7A1–4) and the glycoprotein-associated amino acid transporters (the gpaAT family, SLC7A5–11), also called light chains or catalytic chains of the hetero(di)meric amino acid transporters (HAT). The associated glycoproteins (heavy chains) 4F2hc (CD98) or rBAT (D2, NBAT) form the SLC3 family. Members of the CAT family transport essentially cationic amino acids by facilitated diffusion with differential trans-stimulation by intracellular substrates. In some cells, they may regulate the rate of NO synthesis by controlling the uptake of l-arginine as the substrate for nitric oxide synthase (NOS). The heterodimeric amino acid transporters are, in contrast, quite diverse in terms of substrate selectivity and function (mostly) as obligatory exchangers. Their selectivity ranges from large neutral amino acids (system L) to small neutral amino acids (ala, ser, cys-preferring, system asc), negatively charged amino acid (system xc−) and cationic amino acids plus neutral amino acids (system y+L and b0,+-like). Cotransport of Na+ is observed only for the y+L transporters when they carry neutral amino acids. Mutations in b0,+-like and y+L transporters lead to the hereditary diseases cystinuria and lysinuric protein intolerance (LPI), respectively.

Amino-acid transport by heterodimers of 4F2hc/CD98 and members of a permease family

Amino-acid transport across cellular plasma membranes depends on several parallel-functioning (co-)transporters and exchangers. The widespread transport system L accounts for a sodium-independent exchange of large, neutral amino acids, whereas the system y+L exchanges positively charged amino acids and/or neutral amino acids together with sodium,. The molecular nature of these transporters remains unknown, although expression of the human cell-surface glycoprotein 4F2 heavy chain (h4F2hc; CD98 in the mouse), is known to induce low levels of L- and/or y+L-type transport. This glycoprotein is found in activated lymphocytes, together with an uncharacterized, disulphide-linked lipophilic light chain with an apparent relative molecular mass of 40,000 (Mr 40K),. Here we identify the permease-related protein E16 (ref. 12) as the first light chain of h4F2hc and show that the resulting heterodimeric complex mediates L-type amino-acid transport. The homologous protein from Schistosoma mansoni, SPRM1, also associates covalently with coexpressed h4F2hc glycoprotein, although it induces amino-acid transport of different substrate specificity. The coexpression of h4F2hc is required for surface expression of these permease-related light chains, which belong to a new family of amino-acid transporters that form heterodimers with cell-surface glycoproteins.

Structural Asymmetry of AcrB Trimer Suggests a Peristaltic Pump Mechanism

The AcrA/AcrB/TolC complex spans the inner and outer membranes of Escherichia coli and serves as its major drug-resistance pump. Driven by the proton motive force, it mediates the efflux of bile salts, detergents, organic solvents, and many structurally unrelated antibiotics. Here, we report a crystallographic structure of trimeric AcrB determined at 2.9 and 3.0 angstrom resolution in space groups that allow asymmetry of the monomers. This structure reveals three different monomer conformations representing consecutive states in a transport cycle. The structural data imply an alternating access mechanism and a novel peristaltic mode of drug transport by this type of transporter.

Amino acid transport of y+L‐type by heterodimers of 4F2hc/CD98 and members of the glycoprotein‐associated amino acid transporter family

Amino acid transport across cellular membranes is mediated by multiple transporters with overlapping specificities. We recently have identified the vertebrate proteins which mediate Na+‐independent exchange of large neutral amino acids corresponding to transport system L. This transporter consists of a novel amino acid permease‐related protein (LAT1 or AmAT‐L‐lc) which for surface expression and function requires formation of disulfide‐linked heterodimers with the glycosylated heavy chain of the h4F2/CD98 surface antigen. We show that h4F2hc also associates with other mammalian light chains, e.g. y+LAT1 from mouse and human which are ∼48% identical with LAT1 and thus belong to the same family of glycoprotein‐associated amino acid transporters. The novel heterodimers form exchangers which mediate the cellular efflux of cationic amino acids and the Na+‐dependent uptake of large neutral amino acids. These transport characteristics and kinetic and pharmacological fingerprints identify them as y+L‐type transport systems. The mRNA encoding my+LAT1 is detectable in most adult tissues and expressed at high levels in kidney cortex and intestine. This suggests that the y+LAT1–4F2hc heterodimer, besides participating in amino acid uptake/secretion in many cell types, is the basolateral amino acid exchanger involved in transepithelial reabsorption of cationic amino acids; hence, its defect might be the cause of the human genetic disease lysinuric protein intolerance.

ACE2 links amino acid malnutrition to microbial ecology and intestinal inflammation

Malnutrition affects up to one billion people in the world and is a major cause of mortality. In many cases, malnutrition is associated with diarrhoea and intestinal inflammation, further contributing to morbidity and death. The mechanisms by which unbalanced dietary nutrients affect intestinal homeostasis are largely unknown. Here we report that deficiency in murine angiotensin I converting enzyme (peptidyl-dipeptidase A) 2 (Ace2), which encodes a key regulatory enzyme of the renin-angiotensin system (RAS), results in highly increased susceptibility to intestinal inflammation induced by epithelial damage. The RAS is known to be involved in acute lung failure, cardiovascular functions and SARS infections. Mechanistically, ACE2 has a RAS-independent function, regulating intestinal amino acid homeostasis, expression of antimicrobial peptides, and the ecology of the gut microbiome. Transplantation of the altered microbiota from Ace2 mutant mice into germ-free wild-type hosts was able to transmit the increased propensity to develop severe colitis. ACE2-dependent changes in epithelial immunity and the gut microbiota can be directly regulated by the dietary amino acid tryptophan. Our results identify ACE2 as a key regulator of dietary amino acid homeostasis, innate immunity, gut microbial ecology, and transmissible susceptibility to colitis. These results provide a molecular explanation for how amino acid malnutrition can cause intestinal inflammation and diarrhoea.

Mutations in SLC6A19 , encoding B 0 AT1, cause Hartnup disorder

Hartnup disorder, an autosomal recessive defect named after an English family described in 1956 (ref. 1), results from impaired transport of neutral amino acids across epithelial cells in renal proximal tubules and intestinal mucosa. Symptoms include transient manifestations of pellagra (rashes), cerebellar ataxia and psychosis. Using homozygosity mapping in the original family in whom Hartnup disorder was discovered, we confirmed that the critical region for one causative gene was located on chromosome 5p15 (ref. 3). This region is homologous to the area of mouse chromosome 13 that encodes the sodium-dependent amino acid transporter B0AT1 (ref. 4). We isolated the human homolog of B0AT1, called SLC6A19, and determined its size and molecular organization. We then identified mutations in SLC6A19 in members of the original family in whom Hartnup disorder was discovered and of three Japanese families. The protein product of SLC6A19, the Hartnup transporter, is expressed primarily in intestine and renal proximal tubule and functions as a neutral amino acid transporter.

System L: heteromeric exchangers of large, neutral amino acids involved in directional transport.

Abstract. The plasma membrane transport system L is in many cells the only (efficient) pathway for the import of large branched and aromatic neutral amino acids. The corresponding transporters are hetero(di)mers composed of a catalytic subunit (LAT1 or LAT2=light chain=glycoprotein-associated amino acid transporter) associated covalently with the glycoprotein 4F2hc/CD98 (heavy chain). The tissue distribution of LAT1 suggests that it is involved mainly in transporting amino acids into growing cells and across some endothelial/epithelial secretory barriers, whereas the localization of LAT2 indicates that it is mainly involved in the basolateral efflux step of transepithelial (re)absorptive amino acid transport. However, system L transporters are obligatory amino acid exchangers with 1:1 stoichiometry, with similar (but not identical) intra- and extracellular substrate selectivities and with highly asymmetrical apparent affinities (low affinity inside). Therefore, net directional transport of large, neutral amino acids by system L depends on the parallel expression of a unidirectional transporter with overlapping selectivity (for instance systems A or N) that provides/recycles amino acids that drive system L exchange function. By mediating the regulated flux of these exchange substrates, unidirectional transporters control the activity of system L.

Activation of system L heterodimeric amino acid exchangers by intracellular substrates

System L‐type transport of large neutral amino acids is mediated by ubiquitous LAT1‐4F2hc and epithelial LAT2‐4F2hc. These heterodimers are thought to function as obligatory exchangers, but only influx properties have been studied in some detail up until now. Here we measured their intracellular substrate selectivity, affinity and exchange stoichiometry using the Xenopus oocyte expression system. Quantification of amino acid influx and efflux by HPLC demonstrated an obligatory amino acid exchange with 1:1 stoichiometry. Strong, differential trans‐stimulations of amino acid influx by injected amino acids showed that the intracellular substrate availability limits the transport rate and that the efflux selectivity range resembles that of influx. Compared with high extracellular apparent affinities, LAT1‐ and LAT2‐4F2hc displayed much lower intracellular apparent affinities (apparent Km in the millimolar range). Thus, the two system L amino acid transporters that are implicated in cell growth (LAT1‐4F2hc) and transcellular transport (LAT2‐4F2hc) are obligatory exchangers with relatively symmetrical substrate selectivities but strongly asymmetrical substrate affinities such that the intracellular amino acid concentration controls their activity.

Transcriptional control of sodium transport in tight epithelia by adrenal steroids

ConclusionsThe model for the adrenal steroid action on Na transport in tight epithelia as depicted in Fig. 3A and B dissociates two phases: an early phase during which the pre-existing Na transport machinery is activated and a late phase during which the transport capacity of the machinery is increased. These two sequential phases have been distinguished based on differences in functional aspects of the induced transport, on selective effects of agents interfering with transcriptional regulation and on a correlation of the late response phase with an increase in transport protein synthesis and expression [26, 45, 46, 98, 99, 124]. These observations suggest that a bimodal stimulation of Na transport could involve two different gene networks which are directly (in the physiological meaning) and independently stimulated by the action of the hormone-receptor complex and the following “molecular” cascades (see section Molecular and Physiological Cascades). The relatively clear temporal dissociation of the responses found in experimental situations is probably the consequence of inherent properties of the two networks. Indeed, to generate rapid functional changes, the genes involved in the early response must encode products which have relatively short half-lifes at the mRNA and protein levels. In contrast, the constitutive elements of the Na transport machinery that are increased during the late phase of adrenal steroid action have, as shown for the Na,K-ATPase [82], relatively long half-lifes. Consequently, even though changes in transcription may take place early in the course of the hormonal treatment, they impact on protein synthesis and pools only slowly and after a substantial lag period.On the one hand, ongoing research will soon provide more information on the nature, time course and hormone/receptor specificity of adrenal-steroid-regulated genes. On the other hand, the availability of new technical and molecular tools to study the proteins of the Na transport machinery greatly increases the possibilities for studying its regulation by adrenal steroids. Consequently, it will be a fascinating challenge to relate the data emerging from both approaches, and it appears that only a combination of methods and tools will allow to progressively fill the gap of understanding which still lies between the transcriptional effects and the transport regulation.

Glycoprotein-associated amino acid exchangers: broadening the range of transport specificity

Abstract. Members of the newly discovered glycoprotein-associated amino acid transporter family (gpaAT-family) share a similar primary structure with >40% identity, a predicted 12-transmembrane segment topology and the requirement for association with a glycoprotein (heavy chain) for functional surface expression. Five of the six identified gpaATs (light chains) associate with the surface antigen 4F2 heavy chain (4F2hc = CD98), a ubiquitous plasma membrane protein induced in cell proliferation, and which is also highly expressed at the basolateral surface of amino acid transporting epithelia. The differing tissue localizations of the 4F2hc-associated gpaATs appear to complement each other. As yet, a single gpaAT (b0,+AT) has been shown to associate with rBAT, a 4F2hc-related glycoprotein mainly localized in intestine and kidney luminal brush-border membranes. The transport characteristics of gpaATs have been shown, by expression in heterologous systems, to correspond to the previously described transport systems L, y+L, xc– and bo,+. These (obligatory) exchangers of broad substrate specificity (with the exception of xCT) are expected to equilibrate the concentrations of their substrate amino acids across membranes. Thus, the driving force provided by a transmembrane gradient of one substrate amino acid, such as that generated by a parallel functioning unidirectional transporter, can be used by a gpaAT to fuel the secondary active vectorial transport of other exchangeable species. Vectorial transport of specific amino acids is also promoted by the intrinsic asymmetry of these exchangers. The fact that genetic defects of the epithelial gpaATs b0,+AT and y+LAT1 cause non-type I cystinuria and lysinuric protein intolerance, respectively, demonstrates that these gpaATs perform vectorial secondary and/or tertiary active transport of specific amino acids in vivo.