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High-resolution imaging of maltoporin LamB while quantifying the free-energy landscape and asymmetry of sugar-binding.

E. Mulvihill,Moritz Pfreundschuh,Johannes Thoma,Noah Ritzmann,D. Müller

Published 2019 in Nano letters (Print)

ABSTRACT

Maltoporins are a family of membrane proteins that facilitate the diffusion of hydrophilic molecules and maltosaccharides across the outer membrane of Gram-negative bacteria. Two contradicting models propose the sugar-binding, -uptake and -transport by maltoporins to be either symmetric or asymmetric. Here, we address this contradiction and introduce force-distance curve-based AFM to image single maltoporin LamB trimers in the membrane at sub-nanometer resolution and simultaneously quantify the binding of different maltooligosaccharides. We assay subtle differences of the binding free-energy landscape of maltotriose, maltotetraose and maltopentaose, which quantifies how binding strength and affinity increases with the maltooligosaccharide chain length. The ligand-binding parameters change considerably by mutating the extracellular loop 3, which folds into and constricts the transmembrane pore of LamB. By recording LamB topographs and structurally mapping binding events at sub-nanometer resolution, we observe LamB to preferentially bind maltodextrin from the periplasmic side, which shows sugar-binding and -uptake to be asymmetric. The study introduces AFM as an analytical nanoscopic tool that can differentiate among the factors modulating and models describing the binding and uptake of substrates by membrane proteins.

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