Fig. 1: Schematics of voltage generation in a PL system and location of DEER probe sites.

In (a–c), the arrows’ thickness indicates the concentration gradient of Na⁺ (red), ¹⁴C-Galactose (Gal*, yellow), and K⁺ ions (black). vSGLT is depicted in gray surface representation while lipid molecules are shown in blue. a The PL system in the absence of valinomycin is assumed to have no K⁺ diffusional potential. Incorporation of Valinomycin in the PLs leads to K⁺-selective transmembrane diffusion. b When [K⁺]_in > [K⁺]_out, a negative potential is established in the PLs. c When [K⁺]_out > [K⁺]_in, a positive potential is established in the PL system. d The outward open model and (e) inward-facing crystal structure of vSGLT (PDB ID: 3DH4) with the chosen sites 279/393 (black spheres) to probe the intracellular side. f The outward open model and (g) inward-facing crystal structure of vSGLT showing the sites 313/423 on the extracellular side as black spheres.