Involved in response to osmotic stress (PubMed:7642496, PubMed:9446558, PubMed:10625602, PubMed:19262666, PubMed:21364531, PubMed:24141878). High-affinity glycine betaine-specific uptake system, which couples the uptake of glycine betaine to the symport of two Na(+) ions (PubMed:7642496, PubMed:10625602, PubMed:15134432, PubMed:19262666, PubMed:21364531, PubMed:22940865, PubMed:24141878). Transport is driven both by the Na(+) gradient and by the electrical potential (PubMed:7642496, PubMed:10625602). In addition, functions both as an osmosensor and as an osmoregulator that transduces signal to the catalytic part of the carrier protein, which adapts its activity to the extent of osmotic stress (PubMed:9446558, PubMed:10625602, PubMed:11574473). Uptake is activated by hyperosmotic stress (PubMed:7642496, PubMed:9446558, PubMed:10625602, PubMed:15134432, PubMed:19262666, PubMed:21364531, PubMed:24141878). Osmoresponsive activation is triggered by a change in the internal K(+) concentration (PubMed:11574473, PubMed:15063732). In addition, shows a pronounced chill stimulation, at temperatures around 10 degrees Celsius (PubMed:15995189, PubMed:17693504). Chill activation may be influenced by the membrane lipid composition (PubMed:17693504). Uptake is completely abolished by the uncoupler CCCP, and to a different extent by the ionophores valinomycin and nigericin (PubMed:7642496). Optimum pH is 7.5-8.5. Homotrimer (PubMed:15046983, PubMed:21681199, PubMed:19262666, PubMed:21364531, PubMed:22940865, PubMed:24141878, PubMed:25023443). The monomer can accumulate glycine betaine, but trimerization is required to properly respond to osmotic stress (PubMed:21681199). Constitutively expressed at a basal level of activity (PubMed:7642496). Induced upon hyperosmotic conditions, resulting in an increase of its transport activity (PubMed:7642496, PubMed:17390131). Contains a negatively charged N-terminus and a positively charged C-terminus, which are both involved in sensing and/or transducing osmotic changes to the domain responsible for the translocation of the substrate glycine betaine (PubMed:9446558). The C-terminal domain is directly involved in K(+) sensing or K(+)-dependent activation of BetP (PubMed:15134432). Mutant shows strongly decreased glycine betaine uptake. Belongs to the BCCT transporter (TC 2.A.15) family. Binds 2 Na(+), but the precise binding sites are still uncertain and may change during the transport cycle.