Transmembrane helix I and periplasmic loop 1 of Escherichia coli ProP are involved in osmosensing and osmoprotectant transport.

Osmoregulatory transporters stimulate bacterial growth by mediating osmoprotectant uptake in response to increasing osmotic pressure. The ProP protein of Escherichia coli transports proline and other osmoprotectants. Like LacY, ProP is a member of the major facilitator superfamily and a H(+)-solute symporter.

Periplasmic loops of osmosensory transporter ProP in Escherichia coli are sensitive to osmolality.

ProP is an osmosensory transporter. The activities of ProP and ProP*, a cysteine-less, His(6)-tagged ProP variant, increase with osmotic pressure in cells and proteoliposomes. In proteoliposomes, ProP activity is osmolality-dependent only if the magnitude of the membrane potential (DeltaPsi) exceeds 100 mV.

Structure and function of transmembrane segment XII in osmosensor and osmoprotectant transporter ProP of Escherichia coli.

Escherichia coli transporter ProP acts as both an osmosensor and an osmoregulator. As medium osmolality rises, ProP is activated and mediates H+-coupled uptake of osmolytes like proline. A homology model of ProP with 12-transmembrane (TM) helices and cytoplasmic termini was created, and the protein's topology was substantiated experimentally.

Formation of an antiparallel, intermolecular coiled coil is associated with in vivo dimerization of osmosensor and osmoprotectant transporter ProP in Escherichia coli.

Membrane transporter ProP from Escherichia coli senses extracellular osmolality and responds by mediating the uptake of osmoprotectants such as glycine betaine when osmolality is high. Earlier EPR and NMR studies showed that a peptide replica of the cytoplasmic ProP carboxyl terminus (residues D468-R497) forms a homodimeric, antiparallel, alpha-helical coiled coil in vitro stabilized by electrostatic interactions involving R488. Amino acid replacement R488I disrupted coiled-coil formation by the ProP peptide, elevated the osmolality at which ProP became active, and rendered the osmolality response of ProP transient.

A structural model for the osmosensor, transporter, and osmoregulator ProP of Escherichia coli.

Transporter ProP of Escherichia coli, a member of the major facilitator superfamily (MFS), acts as an osmosensor and an osmoregulator in cells and after purification and reconstitution in proteoliposomes. H(+)-osmoprotectant symport via ProP is activated when medium osmolality is elevated with membrane impermeant osmolytes. The three-dimensional structure of ProP was modeled with the crystal structure of MFS member GlpT as a template.