J. Bacteriol., 10 1997, 6213-6220, Vol 179, No. 20
B Kempf, J Gade and E Bremer
The OpuA transport system of Bacillus subtilis functions as a high-
affinity uptake system for the osmoprotectant glycine betaine. It is a
member of the ABC transporter superfamily and consists of an ATPase
(OpuAA), an integral membrane protein (OpuAB), and a hydrophilic
polypeptide (OpuAC) that shows the signature sequence of lipoproteins (B.
Kempf and E. Bremer, J. Biol. Chem. 270:16701-16713, 1995). The OpuAC
protein might thus serve as an extracellular substrate binding protein
anchored in the cytoplasmic membrane via a lipid modification at an
amino-terminal cysteine residue. A malE-opuAC hybrid gene was constructed
and used to purify a lipidless OpuAC protein. The purified protein bound
radiolabeled glycine betaine avidly and exhibited a KD of 6 microM for this
ligand, demonstrating that OpuAC indeed functions as the substrate binding
protein for the B. subtilis OpuA system. We have selectively expressed the
opuAC gene under T7 phi10 control in Escherichia coli and have demonstrated
through its metabolic labeling with [3H]palmitic acid that OpuAC is a
lipoprotein. A mutant expressing an OpuAC protein in which the
amino-terminal cysteine residue was changed to an alanine (OpuAC-3) was
constructed by oligonucleotide site- directed mutagenesis. The OpuAC-3
protein was not acylated by [3H]palmitic acid, and part of it was secreted
into the periplasmic space of E. coli, where it could be released from the
cells by cold osmotic shock. The opuAC-3 mutation was recombined into an
otherwise wild-type opuA operon in the chromosome of B. subtilis.
Unexpectedly, this mutant OpuAC system still functioned efficiently for
glycine betaine acquisition in vivo under high-osmolarity growth
conditions. In addition, the mutant OpuA transporter exhibited kinetic
parameters similar to that of the wild-type system. Our data suggest that
the lipidless OpuAC-3 protein is held in the cytoplasmic membrane of B.
subtilis via its uncleaved hydrophobic signal peptide.
Copyright © 1997, American Society for Microbiology
Lipoprotein from the osmoregulated ABC transport system OpuA of Bacillus subtilis: purification of the glycine betaine binding protein and characterization of a functional lipidless mutant
Department of Biology, Philipps University Marburg, Germany.
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