Arginine oscillation explains Na+ independence in the substrate/product antiporter CaiT

Many secondary-active transporters use a sodium gradient to translocate their substrate along with a sodium ion or ions across the membrane. In contrast to other, closely related transporters, the carnitine transporter CaiT does not depend on an ion gradient. We show here that the positively charged amino acid sidechain arginine 262 (R262) in CaiT replaces the sodium ion required by other transporters. Mutating R262 in CaiT makes substrate binding and transport sodium dependent. Modeling studies reveal that R262 adopts various orientations in different conformational states of the CaiT transport cycle. We propose that this oscillation of R262 mimics sodium binding and dissociation that is crucial for triggering conformational changes resulting in substrate translocation.