Difference in Sodium Requirement of Branched Chain Amino Acid Carrier between Pseudomonas aeruginosa PAO and PML Strains Is Due to Substitution of an Amino Acid at Position 292

Phospholipids stabilize the secondary structure of the sodium-coupled branched-chain amino acid carrier of Pseudomonas aeruginosa

For functional reconstitution of bacterial cotransporters (carriers or permeases) including the sodium-coupled branched-chain amino acid carrier (LIV-II carrier) of Pseudomonas aeruginosa, the presence of phospholipid is required through the process of solubilization and purification of the transporters from the bacterial membranes, suggesting the possibility that phospholipid may stabilize the structure of the cotransporter proteins to be in a functional form. In this study, this possibility was examined by studying the effect of denaturant on the secondary structure of the LIV-II carrier purified in the absence and presence of phospholipid using circular dichroism (CD) spectroscopy. CD spectra of the purified LIV-II carrier solubilized in n-octyl-beta-D-glucopyranoside (OG), OG/dioleoylphosphatidylethanolamine (DOPE)/dioleoylphosphatidylglycerol (DOPG) mixture, and dispersed into DOPE/DOPG small unilamellar vesicles were measured in the absence of denaturant. The three spectra were very similar and had a trough at 222 nm with mean residue molar ellipticity of -23000 deg.cm(2)/dmol and a shoulder at 208 nm. CD spectral analyses with three different methods (S.W. Provencher, J. Glöckner, Estimation of globular protein secondary structure from circular dichroism, Biochemistry 20 (1981) 33-37; J.Y. Yang, C.-S.C. Wu, H.Z. Martinez, Calculation of protein conformation from circular dichroism, Methods Enzymol. 130 (1986) 208-269; N. Sreerama, R.W. Woody, A self-consistent method for the analysis of protein secondary structure from circular dichroism, Anal. Biochem. 209 (1993) 32-44) revealed that the LIV-II carrier solubilized in OG/DOPE/DOPG mixture contained 69-75% alpha-helix and 0-9% beta-sheet. Addition of 6 M guanidine hydrochloride decreased 48% of the amplitude at 222 nm of the CD spectrum of the carrier solubilized in OG alone and 9-14% of the CD amplitude of the carrier solubilized in OG/DOPE/DOPG or OG/dioleoylphosphatidylcholine mixture and dispersed in liposomes composed of DOPE/DOPG. These results show that the ordered secondary structure of the LIV-II carrier is partially unfolded in OG without phospholipid by denaturant but is greatly stabilized with phospholipids with oleoyl chains independently of their polar head group composition and suggest that the alpha-helical structure of the carrier is mainly embedded in the lipid environment.

A functional superfamily of sodium/solute symporters

Eleven families of sodium/solute symporters are defined based on their degrees of sequence similarities, and the protein members of these families are characterized in terms of their solute and cation specificities, their sizes, their topological features, their evolutionary relationships, and their relative degrees and regions of sequence conservation. In some cases, particularly where site-specific mutagenesis analyses have provided functional information about specific proteins, multiple alignments of members of the relevant families are presented, and the degrees of conservation of the mutated residues are evaluated. Signature sequences for several of the eleven families are presented to facilitate identification of new members of these families as they become sequenced. Phylogenetic tree construction reveals the evolutionary relationships between members of each family. One of these families is shown to belong to the previously defined major facilitator superfamily. The other ten families do not show sufficient sequence similarity with each other or with other identified transport protein families to establish homology between them. This study serves to clarify structural, functional and evolutionary relationships among eleven distinct families of functionally related transport proteins.

Isolation of the braZ gene encoding the carrier for a novel branched-chain amino acid transport system in Pseudomonas aeruginosa PAO

The braZ gene for a novel branched-chain amino acid transport system in Pseudomonas aeruginosa PAO was isolated and characterized. Determination of the nucleotide sequence showed that the braZ gene comprises 1,311 nucleotides specifying a protein of 437 amino acids. Hydropathy analysis suggested that the product is an integral membrane protein with 12 membrane-spanning segments. The amino acid sequence showed extensive homology to those of the braB and brnQ gene products, branched-chain amino acid carriers of P. aeruginosa and Salmonella typhimurium, respectively. By using the T7 RNA polymerase-promoter system, the braZ gene product was identified as a protein of an apparent Mr of 34,000 on a sodium dodecyl sulfate-polyacrylamide gel. Properties of the transport system encoded by braZ were studied by using P. aeruginosa PAO3537, defective in both the high- and low-affinity branched-chain amino acid transport systems (LIV-I and LIV-II, respectively). The transport system encoded by braZ was found to be another effective branched-chain amino acid transport system in P. aeruginosa PAO and was thus designated as LIV-III. This system is specific for isoleucine and valine, giving the same Km value of 12 microM for these amino acids. The system was found, however, to have a very low affinity for leucine, with a Km value of 150 microM, which contrasts with the substrate specificities of LIV-I and LIV-II.

Cloning and nucleotide sequence of the gene braB coding for the sodium-coupled branched-chain amino acid carrier in Pseudomonas aeruginosa PAO

The gene braB, encoding the Na(+)-coupled carrier for branched-chain amino acids in Pseudomonas aeruginosa PAO, was cloned on cosmid pMMB34. The cosmid clones carrying the braB gene were identified as those that restored growth at low leucine concentration and Na(+)-dependent leucine transport activity to P. aeruginosa PAO3536 defective in the transport of branched-chain amino acids. Determination of the nucleotide sequence of the DNA fragment shows that the braB gene comprises 1311 bp and encodes a hydrophobic protein of 437 amino acids with a calculated Mr of 45,279. The hydropathy profile suggests that there exist in the carrier protein 12 hydrophobic segments long enough to traverse the membrane. The amino acid sequence shows a high degree of homology with the brnQ product, a branched-chain amino acid carrier of Salmonella typhimurium, while no homology in the nucleotide sequences is found in the braB and brnQ genes.