High-affinity Na(+)-coupled glycine betaine and proline betaine transport. Required for a rapid adaptation to osmotic stress. Does not transport other trimethylammonium or related molecules, such as choline, carnitine and ectoine. Uptake is stimulated by high osmolarity. Stimulation is probably due to conformational changes. Constitutively expressed. Inactivation results in loss of protection by both glycine betaine and proline betaine after an osmotic upshock. Belongs to the BCCT transporter (TC 2.A.15) family.