Structural and functional importance of transmembrane domain 3 (TM3) in the aspartate: alanine antiporter AspT: Topology and function of the residues of TM3 and oligomerization of AspT

Kei Nanatani, Peter C. Maloney, Keietsu Abe

Research output: Contribution to journalArticle

Abstract

AspT, the aspartate:alanine antiporter of Tetragenococcus halophilus, a membrane protein of 543 amino acids with 10 putative transmembrane (TM) helices, is the prototype of the aspartate:alanine exchanger (AAE) family of transporters. Because TM3 (isoleucine 64 to methionine 85) has many amino acid residues that are conserved among members of the AAE family and because TM3 contains two charged residues and four polar residues, it is thought to be located near (or to form part of) the substrate translocation pathway that includes the binding site for the substrates. To elucidate the role of TM3 in the transport process, we carried out cysteine-scanning mutagenesis. The substitutions of tyrosine 75 and serine 84 had the strongest inhibitory effects on transport (initial rates of L-aspartate transport were below 15% of the rate for cysteine-less AspT). Considerable but less-marked effects were observed upon the replacement of methionine 70, phenylalanine 71, glycine 74, arginine 76, serine 83, and methionine 85 (initial rates between 15% and 30% of the rate for cysteine-less AspT). Introduced cysteine residues at the cytoplasmic half of TM3 could be labeled with Oregon green maleimide (OGM), whereas cysteines close to the periplasmic half (residues 64 to 75) were not labeled. These results suggest that TM3 has a hydrophobic core on the periplasmic half and that hydrophilic residues on the cytoplasmic half of TM3 participate in the formation of an aqueous cavity in membranes. Furthermore, the presence of L-aspartate protected the cysteine introduced at glycine 62 against a reaction with OGM. In contrast, L-aspartate stimulated the reactivity of the cysteine introduced at proline 79 with OGM. These results demonstrate that TM3 undergoes L-aspartate-induced conformational alterations. In addition, nonreducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis analyses and a glutaraldehyde cross-linking assay suggest that functional AspT forms homo-oligomers as a functional unit.

Original languageEnglish (US)
Pages (from-to)2122-2132
Number of pages11
JournalJournal of Bacteriology
Volume191
Issue number7
DOIs
StatePublished - Apr 2009

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Antiporters
Aspartic Acid
Alanine
Cysteine
Methionine
Glycine
Serine
Enterococcaceae
Amino Acids
Isoleucine
Glutaral
Phenylalanine
Proline
Mutagenesis
Sodium Dodecyl Sulfate
Tyrosine
Arginine
Polyacrylamide Gel Electrophoresis
Membrane Proteins
Binding Sites

ASJC Scopus subject areas

  • Microbiology
  • Molecular Biology

Cite this

Structural and functional importance of transmembrane domain 3 (TM3) in the aspartate : alanine antiporter AspT: Topology and function of the residues of TM3 and oligomerization of AspT. / Nanatani, Kei; Maloney, Peter C.; Abe, Keietsu.

In: Journal of Bacteriology, Vol. 191, No. 7, 04.2009, p. 2122-2132.

Research output: Contribution to journalArticle

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abstract = "AspT, the aspartate:alanine antiporter of Tetragenococcus halophilus, a membrane protein of 543 amino acids with 10 putative transmembrane (TM) helices, is the prototype of the aspartate:alanine exchanger (AAE) family of transporters. Because TM3 (isoleucine 64 to methionine 85) has many amino acid residues that are conserved among members of the AAE family and because TM3 contains two charged residues and four polar residues, it is thought to be located near (or to form part of) the substrate translocation pathway that includes the binding site for the substrates. To elucidate the role of TM3 in the transport process, we carried out cysteine-scanning mutagenesis. The substitutions of tyrosine 75 and serine 84 had the strongest inhibitory effects on transport (initial rates of L-aspartate transport were below 15{\%} of the rate for cysteine-less AspT). Considerable but less-marked effects were observed upon the replacement of methionine 70, phenylalanine 71, glycine 74, arginine 76, serine 83, and methionine 85 (initial rates between 15{\%} and 30{\%} of the rate for cysteine-less AspT). Introduced cysteine residues at the cytoplasmic half of TM3 could be labeled with Oregon green maleimide (OGM), whereas cysteines close to the periplasmic half (residues 64 to 75) were not labeled. These results suggest that TM3 has a hydrophobic core on the periplasmic half and that hydrophilic residues on the cytoplasmic half of TM3 participate in the formation of an aqueous cavity in membranes. Furthermore, the presence of L-aspartate protected the cysteine introduced at glycine 62 against a reaction with OGM. In contrast, L-aspartate stimulated the reactivity of the cysteine introduced at proline 79 with OGM. These results demonstrate that TM3 undergoes L-aspartate-induced conformational alterations. In addition, nonreducing sodium dodecyl sulfate-polyacrylamide gel electrophoresis analyses and a glutaraldehyde cross-linking assay suggest that functional AspT forms homo-oligomers as a functional unit.",
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