Aquaporins (AQPs) are a ubiquitous family of transmembrane water channel proteins. A subgroup of AQP water channels also facilitates transmembrane diffusion of small, polar solutes. A constriction within the pore, the aromatic/arginine (ar/R) selectivity filter, is thought to control solute permeability: previous studies on single representative water channel proteins suggest narrow channels conduct water, whilst wider channels permit passage of solutes. To assess this model of selectivity, we used mutagenesis, permeability measurements and in silico comparisons of water-specific as well as glycerol-permeable human AQPs. Our studies show that single amino acid substitutions in the selectivity filters of AQP1, AQP4 and AQP3 differentially affect glycerol and urea permeability in an AQP-specific manner. Comparison between in silico-calculated channel cross-sectional areas and in vitro permeability measurements suggests that selectivity filter cross-sectional area predicts urea but not glycerol permeability. Our data show that substrate discrimination in water channels depends on a complex interplay between the solute, pore size, and polarity, and that using single water channel proteins as representative models has led to an underestimation of this complexity.
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Funding: PK was supported
by Aston University through a 50th Anniversary Prize Fellowship and by the Engineering and Physical Sciences
Research Council through the Molecular Organisation and Assembly in Cells Doctoral Training Centre,
University of Warwick, grant number EP/F500378/1. RMB, ACC and PK acknowledge International Partnering
Award, BB BB/P025927/1, from the UK Biotechnology and Biosciences Research Council. MMS was supported
by HCED grant number GD-13-13 (M. Salman). SUP was supported by the European Union Erasmus exchange
Funding: Engineering and Physical Sciences Research Council EP/F500378/1, UK Biotechnology and Biosciences Research Council BB BB/P025927/1, HCED grant number GD-13-13
Kitchen, P., Salman, M. M., Pickel, S., Jennings, J., Törnroth-horsefield, S., Conner, M., Bill, R., & Conner, A. C. (2019). Water channel pore size determines exclusion properties but not solute selectivity. Scientific Reports, 9(1), . https://doi.org/10.1038/s41598-019-56814-z