Mechanism of proline transport in Escherichia coli K12. II. Effect of alkaline cations on binding of proline to a H+/proline symport carrier in cytoplasmic membrane vesicles

Identification of a second substrate-binding site in solute-sodium symporters

The structure of the sodium/galactose transporter (vSGLT), a solute-sodium symporter (SSS) from Vibrio parahaemolyticus, shares a common structural fold with LeuT of the neurotransmitter-sodium symporter family. Structural alignments between LeuT and vSGLT reveal that the crystallographically identified galactose-binding site in vSGLT is located in a more extracellular location relative to the central substrate-binding site (S1) in LeuT. Our computational analyses suggest the existence of an additional galactose-binding site in vSGLT that aligns to the S1 site of LeuT. Radiolabeled galactose saturation binding experiments indicate that, like LeuT, vSGLT can simultaneously bind two substrate molecules under equilibrium conditions. Mutating key residues in the individual substrate-binding sites reduced the molar substrate-to-protein binding stoichiometry to ~1. In addition, the related and more experimentally tractable SSS member PutP (the Na(+)/proline transporter) also exhibits a binding stoichiometry of 2. Targeting residues in the proposed sites with mutations results in the reduction of the binding stoichiometry and is accompanied by severely impaired translocation of proline. Our data suggest that substrate transport by SSS members requires both substrate-binding sites, thereby implying that SSSs and neurotransmitter-sodium symporters share common mechanistic elements in substrate transport.

Mutant LV(476-7)AA of A-subunit of Enterococcus hirae V1-ATPase: High affinity of A3B3 complex to DF axis and low ATPase activity

Vacuolar ATPase (V-ATPase) of Enterococcus hirae is composed of a soluble functional domain V₁ (A₃B₃DF) and an integral membrane domain V_o (ac), where V₁ and V_o domains are connected by a central stalk, composed of D-, F-, and d-subunits; and two peripheral stalks (E- and G-subunits). We identified 120 interacting residues of A₃B₃ heterohexamer with D-subunit in DF heterodimer in the crystal structures of A₃B₃ and A₃B₃DF. In our previous study, we reported 10 mutants of E. hirae V₁-ATPase, which showed lower binding affinities of DF with A₃B₃ complex leading to higher initial specific ATPase activities compared to the wild-type. In this study, we identified a mutation of A-subunit (LV^476-7AA) at its C-terminal domain resulting in the A₃B₃ complex with higher binding affinities for wild-type or mutant DF heterodimers and lower initial ATPase activities compared to the wild-type A₃B₃ complex, consistent with our previous proposal of reciprocal relationship between the ATPase activity and the protein-protein binding affinity of DF axis to the A₃B₃ catalytic domain of E. hirae V-ATPase. These observations suggest that the binding of DF axis at the contact region of A₃B₃ rotary ring is relevant to its rotation activity.

Loose binding of the DF axis with the A3B3 complex stimulates the initial activity of Enterococcus hirae V1-ATPase

Vacuolar ATPases (V-ATPases) function as proton pumps in various cellular membrane systems. The hydrophilic V1 portion of the V-ATPase is a rotary motor, in which a central-axis DF complex rotates inside a hexagonally arranged catalytic A3B3 complex by using ATP hydrolysis energy. We have previously reported crystal structures of Enterococcushirae V-ATPase A3B3 and A3B3DF (V1) complexes; the result suggested that the DF axis induces structural changes in the A3B3 complex through extensive protein-protein interactions. In this study, we mutated 10 residues at the interface between A3B3 and DF complexes and examined the ATPase activities of the mutated V1 complexes as well as the binding affinities between the mutated A3B3 and DF complexes. Surprisingly, several V1 mutants showed higher initial ATPase activities than wild-type V1-ATPase, whereas these mutated A3B3 and DF complexes showed decreased binding affinities for each other. However, the high ATP hydrolysis activities of the mutants decreased faster over time than the activity of the wild-type V1 complex, suggesting that the mutants were unstable in the reaction because the mutant A3B3 and DF complexes bound each other more weakly. These findings suggest that strong interaction between the DF complex and A3B3 complex lowers ATPase activity, but also that the tight binding is responsible for the stable ATPase activity of the complex.

Reconstitution in vitro of the catalytic portion (NtpA3-B3-D-G complex) of Enterococcus hirae V-type Na+-ATPase

Enterococcus hirae vacuolar ATPase (V-ATPase) is composed of a soluble catalytic domain (V(1); NtpA(3)-B(3)-D-G) and an integral membrane domain (V(0); NtpI-K(10)) connected by a central and peripheral stalk(s) (NtpC and NtpE-F). Here we examined the nucleotide binding of NtpA monomer, NtpB monomer or NtpD-G heterodimer purified by using Escherichia coli expression system in vivo or in vitro, and the reconstitution of the V(1) portion with these polypeptides. The affinity of nucleotide binding to NtpA was 6.6 microM for ADP or 3.1 microM for ATP, while NtpB or NtpD-G did not show any binding. The NtpA and NtpB monomers assembled into NtpA(3)-B(3) heterohexamer in nucleotide binding-dependent manner. NtpD-G bound NtpA(3)-B(3) forming V(1) (NtpA(3)-B(3)-D-G) complex independent of nucleotides. The V(1) formation from individual NtpA and NtpB monomers with NtpD-G heterodimer was absolutely dependent on nucleotides. The ATPase activity of reconstituted V(1) complex was as high as that of native V(1)-ATPase purified from the V(0)V(1) complex by EDTA treatment of cell membrane. This in vitro reconstitution system of E. hirae V(1) complex will be valuable for characterizing the subunit-subunit interactions and assembly mechanism of the V(1)-ATPase complex.

Na+ binding of V-type Na+-ATPase in Enterococcus hirae

Rotation catalysis theory has been successfully applied to the molecular mechanism of the ATP synthase (F(0)F(1)-ATPase) and probably of the vacuolar ATPase. We investigated the ion binding step to Enterococcus hirae Na(+)-translocating V-ATPase. The kinetics of Na(+) binding to purified V-ATPase suggested 6 +/- 1 Na(+) bound/enzyme molecule, with a single high affinity (K(d(Na(+()))) = 15 +/- 5 micrometer). The number of cation binding sites is consistent with the model that V-ATPase proteolipids form a rotor ring consisting of hexamers, each having one cation binding site. Release of the bound (22)Na(+) from purified molecules in a chasing experiment showed two phases: a fast component (about two-thirds of the total amount of bound Na(+); k(exchange) > 1.7 min(-1)) and a slow component (about one-third of the total; k(exchange) = 0.16 min(-1)), which changes to the fast component by adding ATP or ATPgammaS. This suggested that about two-thirds of the Na(+) binding sites of the Na(+)-ATPase are readily accessible from the aqueous phase and that the slow component is important for the transport reaction.

Sodium ion and proline binding sites in the Na+/proline symport carrier of Escherichia coli

Proline binding activity of the Escherichia coli Na+/proline symport carrier is inhibited by a sulfhydryl reagent, N-ethylmaleimide (NEM). Proline and its analogs protected the carrier against the NEM-inactivation in a Na+ (or Li+)-dependent manner. Na+ alone, even in the absence of proline, partially protected it from the NEM-inactivation. Mutant proline carriers, CS281, CS344 and CS349, which have a serine residue in place of Cys-281, Cys-344 and Cys-349, respectively (Yamato, I. and Anraku, Y. (1988) J. Biol. Chem. 263, 16055-16057) were also analyzed for cation-dependent proline binding and NEM-sensitivity. Proline binding activities of CS281 and CS344 were almost completely resistant to NEM, whereas that of CS349 was not. Furthermore, the proline binding activity of CS344 was remarkably lower than those of the wild-type, CS281 and CS349 carriers. These results indicate that Cys-344, which is located in the putative eighth membrane-spanning domain in the carrier, is a cysteine residue functionally involved in the high-affinity binding for sodium ion and proline.

Ordered binding model as a general mechanistic mechanism for secondary active transport systems

The mechanistic mechanism of secondary active transport processes has not been fully elucidated. Based on substrate binding studies dependent on coupling cation concentrations of the glutamate, melibiose, lactose and proline transport carriers in Escherichia coli, the ordered binding mechanism was proposed as the energy coupling mechanism of the transport systems. This ordered binding mechanism satisfactorily explained the properties of the secondary active transport systems. Thus, this mechanism as the general energy coupling mechanism for the transport systems is discussed.

Mechanism of Na+/proline symport in Escherichia coli: reappraisal of the effect of cation binding to the Na+/proline symport carrier

The proton and sodium ion dependences of the proline binding and transport activities of the proline carrier in Escherichia coli were investigated in detail. The binding activity in cytoplasmic membrane vesicles from a carrier over-producing strain (PT21/pTMP5) was absolutely dependent on the presence of Na+, but did not necessarily require protonation of the carrier, in contrast to the model previously reported (Mogi, T., Anraku, Y. 1984. J. Biol. Chem. 259:7797-7801). Based on this and previous observations, we propose a modified model of the proline binding reaction of the proline carrier, in which a proton is supposed to be a regulatory factor for the binding activity. The apparent Michaelis constant of proline (Kt) of the transport activity of cytoplasmic membrane vesicles from the wild type E. coli strain driven by a respiratory substrate, ascorbate, showed dependence on a low concentration of sodium ion. The Michaelis constant of sodium ion for transport (KtNa) was estimated to be 25 microM. The proline transport activities in membrane vesicles and intact cells were modulated by H+ concentration, the inhibitory effect of protons (pKa approximately equal to 6) being similar to that observed previously (Mogi, T., Anraku, Y. 1984. J. Biol. Chem. 259:7802-7806). Based on these observations and the modified model of substrate binding to the proline carrier, a model of the proline/Na+ symport mechanism is proposed, in which a proton is postulated to be a regulatory factor of the transport activity.

Molecular cloning of gltS and gltP, which encode glutamate carriers of Escherichia coli B

Two genes encoding distinct glutamate carrier proteins of Escherichia coli B were cloned into an E. coli K-12 strain by using a cosmid vector, pHC79. One of them was the gltS gene coding for a glutamate carrier of an Na+-dependent, binding protein-independent, and glutamate-specific transport system. The content of the glutamate carrier was amplified about 25-fold in the cytoplasmic membranes from a gltS-amplified strain. The gltS gene was located in a 3.2-kilobase EcoRI-MluI fragment, and the gene product was identified as a membrane protein with an apparent Mr of 35,000 in a minicell system. A gene designated gltP was also cloned. The transport activity of the gltP system in cytoplasmic membrane vesicles from a gltP-amplified strain was driven by respiratory substrates and was independent of the concentrations of Na+, K+, and Li+. An uncoupler, carbonylcyanide m-chlorophenylhydrazone, completely inhibited the transport activities of both systems, whereas an ionophore, monensin, inhibited only that of the gltS system. The Kt value for glutamate was 11 microM in the gltP system and 3.5 microM in the gltS system. L-Aspartate inhibited the glutamate transport of the gltP system but not that of the gltS system. Aspartate was taken up actively by membrane vesicles from the gltP-amplified strain, although no aspartate uptake activity was detected in membrane vesicles from a wild-type E. coli strain. These results suggest that gltP is a structural gene for a carrier protein of an Na+-independent, binding protein-independent glutamate-aspartate transport system.

Proline porters effect the utilization of proline as nutrient or osmoprotectant for bacteria

Proline is utilized by all organisms as a protein constituent. It may also serve as a source of carbon, energy and nitrogen for growth or as an osmoprotectant. The molecular characteristics of the proline transport systems which mediate the multiple functions of proline in the Gram negative enteric bacteria, Escherichia coli and Salmonella typhimurium, are now becoming apparent. Recent research on those organisms has provided both protocols for the genetic and biochemical characterization of the enzymes mediating proline transport and molecular probes with which the degree of homology among the proline transport systems of archaebacteria, eubacteria and eukaryotes can be assessed. This review has provided a detailed summary of recent research on proline transport in E. coli and S. typhimurium; the properties of other organisms are cited primarily to illustrate the generality of those observations and to show where homologous proline transport systems might be expected to occur. The characteristics of proline transport in eukaryotic microorganisms have recently been reviewed (Horak, 1986).

Proline carrier mutant of Escherichia coli K-12 with altered cation sensitivity of substrate-binding activity: cloning, biochemical characterization, and identification of the mutation

Two putP mutants of Escherichia coli K-12 that were defective in proline transport but retained the binding activities of the major proline carrier were isolated (T. Mogi, H. Yamamoto, T. Nakao, I. Yamato, and Y. Anraku, Mol. Gen. Genet. 202:35-41, 1986). One of these mutations and three null-type mutations (K. Motojima, I. Yamato, and Y. Anraku, J. Bacteriol. 136:5-9, 1978) were cloned into a pBR322 putP+ hybrid plasmid (pTMP5) by in vivo recombination. Cytoplasmic membrane vesicles were prepared from the mutant strains and strains harboring pTMP5 putP plasmids, and the properties of the proline-binding reaction of the mutant putP carriers in membranes were examined under nonenergized conditions. The putP19, putP21, and putP22 mutations, which were mapped in the same DNA segment of the putP gene (Mogi et al., Mol. Gen. Genet. 202:35-41, 1986), caused the complete loss of proline carrier activity. The proline carriers encoded by the mutant putP genes, putP9 and putP32, and putP32 in pTMP5-32, which was derived from in vivo recombination with the putP32 mutation, had altered sodium ion and proton dependence of binding affinities for proline and were resistant to N-ethylmaleimide inactivation without changes in the specificities for substrates and alkaline metal cations. The nucleotide sequence of the putP32 lesion located on the 0.35-megadalton RsaI-PvuII fragment in the putP gene in pTMP5-32 was determined; the mutation changed a cytosine at position 1001 to a thymine, causing the alteration of arginine to cysteine at amino acid position 257 in the primary structure of the proline carrier. It was shown that this one point mutation was enough to produce the phenotype of pTMP5-32 by in vitro DNA replacement of the AcyI-PvuII fragment of the wild-type putP gene with the DNA fragment containing the mutated nucleotide sequence.

Solubilization and reconstitution of proline carrier in Escherichia coli; quantitative analysis and optimal conditions

Proline carrier of Escherichia coli was extracted from the carrier-overproducing membranes with dodecylmaltoside in the presence of phospholipid. The solubilized carrier showed the same proline binding activity as that in normal membranes. As judged from determinations of the binding activity in the micellar state as a marker of active carrier and the radioactivity of N-[ethyl-2-3H]ethylmaleimide-labeled carrier as a marker of carrier polypeptide, 80% of the carrier molecules in the membranes were extracted. Optimal conditions for reconstitution of the solubilized carrier were established. By a combination of freeze-thawing, sonication and dilution procedures, 70% of the solubilized carrier molecules were incorporated into proteoliposomes and the restored active transport of proline showed an apparent Kt of 1 microM and turnover number of 0.6 s-1. The transport of proline was driven by a membrane potential in a Na+ (or Li+)-dependent manner.