Single-molecule AFM studies of substrate transport by using the sodium-glucose cotransporter SGLT1

Abstract

In an apical membrane of epithelial cells from the small intestine and the kidney, the high-affinity Na+/D-glucose cotransporter type 1 (SGLT1) plays a crucial role in intestinal glucose absorption and in renal glucose reabsorption. Here, the over-expression of rabbit SGLT1 in rbSGLT1-transfected Chinese hamster ovary (CHO) cells was first characterized using the immuno-staining method on non-permeabilized cells. The cells were then imaged with atomic force microscopy (AFM), revealing live and fixed cells strongly attached to the glass surfaces. A bioconjugate chemistry approach was employed to functionalize the surfaces of the AFM tips with D-glucose molecules via three different heterobifunctional crosslinkers. The D-glucose binding site and the translocation pathway of SGLT1 were investigated by studying the interaction forces between tip-bound D-glucose and SGLT1 in live cells on the single-molecule level. Analysis of these forces suggested that a long crosslinker with a small end group might be suitable for probing the D-glucose transport pathway of SGLT1. We show that single-molecule AFM technology is a powerful method for investigating transmembrane proteins and transporter functions in live cells.