1 Department of Physics, Concordia University, Montreal QC H4B 1R6, Canada.
2 Centre for Research in Molecular Modeling, Concordia University, Montreal, Quebec H4B 1R6, Canada.
3 Department of Molecular and Cellular Biology, University of Guelph, Guelph, Ontario N1G 2W1, Canada.
4 Department of Chemistry and Center for Computational and Integrative Biology, Rutgers University, Camden, New Jersey 08102, United States.

Osmosensing transporter ProP detects the increase in cytoplasmic cation concentration associated with osmotically induced cell dehydration and mediates osmolyte uptake into bacteria. ProP is a 12-transmembrane helix protein with an a-helical, cytoplasmic C-terminal domain (CTD) linked to transmembrane helix XII (TM XII). It has been proposed that the CTD helix associates with the anionic membrane surface to lock ProP in an inactive conformation and that the release of the CTD may activate ProP. To investigate this possible activation mechanism, we have built and simulated a structural model in which the CTD was anchored to the membrane by TM XII and the CTD helix was associated with the membrane surface. Molecular dynamics simulations showed specific intrapeptide salt bridges forming when the CTD associated with the membrane. Experiments supported the presence of the salt bridge Lys447-Asp455 and suggested a role for these residues in osmosensing. Simulations performed at different salt concentrations showed weakened CTD-lipid interactions at 0.25 M KCl and gradual stiffening of the membrane with increasing salinity. These results suggest that salt cations may affect CTD release and activate ProP by increasing the order of membrane phospholipids.