Sidedness and accessibility of protein epitopes in intact brush border membrane vesicles were analyzed by detecting single molecule interaction forces using molecular recognition force microscopy in aqueous physiological solutions. Frequent antibody-antigen recognition events were observed with a force microscopy tip carrying an antibody directed against the periplasmically located gamma-glutamyltrans- peptidase, suggesting a right side out orientation of the vesicles. Phlorizin attached to the tips bound to NA+/D-glucose cotransporter molecules present in the vesicles. The recognition was sodium dependent and inhibited by free phlorizin and D-glucose, and revealed an apparent K(D) of 0.2 microM. Binding events were also observed with an antibody directed against the epitope aa603-aa630 close to the C terminus of the transporter. In the presence of phlorizin the probability of antibody binding was reduced but the most probable unbinding force f(u) = 100 pN remained unchanged. In the presence of D-glucose and sodium, however, both the binding probability and the most probable binding force (f(u) = 50 pN) were lower than in its absence. These studies demonstrate that molecular recognition force microscopy is a versatile tool to probe orientation and conformational changes of epitopes of membrane components during binding and trans-membrane transport.

Oriented stable binding of functional proteins on surfaces is of fundamental interest for receptor/ligand studies in atomic force microscopy (AFM) and surface plasmon resonance (SPR) experiments. Here we have chosen the His6-tagged carboxyl-tail (C-tail) of the α1C-subunit of the L-type Ca2+ channel and calmodulin (CaM) as its cognitive partner as a model system to develop a new functional surface. Covalently attached self-assembled monolayers on ultraflat gold containing NTA-thiols to which the His6-tagged C-tail was bound and thiols with triethylene-glycol groups as matrix-thiols represented the system of choice. The topography of this surface was characterized using AFM; its ability to bind C-tail proteins oriented and stable was confirmed by SPR measurements and by complementary force spectroscopy experiments with a CaM4-construct covalently attached to the tip. The developed anchoring strategy can now be used to study receptor/ligand interactions in general applying force spectroscopy and SPR on His6-tagged proteins oriented immobilized onto this new NTA-functionalized self-assembled monolayer.

In the apical membrane of epithelial cells from the small intestine and the kidney, the high-affinity Na+/d-glucose cotransporter SGLT1 plays a crucial role in selective sugar absorption and reabsorption. How sugars are selected at the molecular level is, however, poorly understood. Here atomic force microscopy (AFM) was employed to investigate the substrate specificity of rbSGLT1 on the single-molecule level, while competitive-uptake assays with isotope-labeled sugars were performed in the study of the stereospecificity of the overall transport. rbSGLT1-transfected Chinese hamster ovary (CHO) cells were used for both approaches. Evidence of binding of d-glucose to the extracellular surface of rbSGLT1 could be obtained using AFM tips carrying 1-thio-d-glucose coupled at the C1 position to a PEG linker via a vinylsulfon group. Competition experiments with monosaccharides in solution revealed the following selectivity ranking of binding: 2-deoxy-d-glucose >or= 6-deoxy-d-glucose > d-glucose > d-galactose >or= alpha-methyl glucoside; 3-deoxy-d-glucose, d-xylose, and l-glucose did not measurably affect binding. These results were different from those of competitive alpha-methyl glucoside transport assays, where the ranking of inhibition was as follows: d-glucose > d-galactose > 6-deoxy-d-glucose; no uptake inhibition by d-xylose, 3-deoxy-d-glucose, 2-deoxy-d-glucose, or l-glucose was observed. Taken together, these results suggest that the substrate specificity of SGLT1 is determined by different recognition sites: one possibly located at the surface of the transporter and others located close to or within the translocation pathway.

Proteins mediating the transport of solutes across the cell membrane control the intracellular conditions in which life can occur. Because of the particular arrangement of spanning a lipid bilayer and the many conformations required for their function, transport proteins pose significant obstacles for the investigation of their structure-function relation. Crystallographic studies, if available, define the transmembrane segments in a "frozen" state and do not provide information on the dynamics of the extramembranous loops, which are similarly evolutionary conserved and thus as functionally important as the other parts of the protein. The current review presents biophysical methods that can shed light on the dynamics of transporters in the membrane. The techniques that are presented in some detail are single-molecule recognition atomic force microscopy and tryptophan scanning, which can report on the positioning of the loops and on conformational changes at the outer surface. Studies on a variety of symporters are discussed, which use gradients of sodium or protons as energy source to translocate (mainly organic) solutes against their concentration gradients into or out of the cells. Primarily, investigations of the sodium-glucose cotransporter SGLT1 are used as examples for this biophysical approach to understand transporter function.

To investigate whether the C-terminal loop 13 of rabbit sodium/glucose cotransporter SGLT1 is involved in the recognition of the substrate D-glucose, isolated loop 13 (amino acids (aa) 541-638) was immobilized to a lipid bilayer. Interactions were investigated by surface plasmon resonance spectroscopy using an antibody directed against the late part of the loop (aa 606 -631) or the glucoside transport inhibitor phlorizin.