title: Enhancement of proton conductance by mutations of the selectivity filter of aquaporin-1.
abstract: Prevention of cation permeation in wild-type aquaporin-1 (AQP1) is believed to be associated with the Asn-Pro-Ala (NPA) region and the aromatic/arginine selectivity filter (SF) domain. Previous work has suggested that the NPA region helps to impede proton permeation due to the protein backbone collective macrodipoles that create an environment favoring a directionally discontinuous channel hydrogen-bonded water chain and a large electrostatic barrier. The SF domain contributes to the proton permeation barrier by a spatial restriction mechanism and direct electrostatic interactions. To further explore these various effects, the free-energy barriers and the maximum cation conductance for the permeation of various cations through the AQP1-R195V and AQP1-R195S mutants are predicted computationally. The cations studied included the hydrated excess proton that utilizes the Grotthuss shuttling mechanism, a model "classical" charge localized hydronium cation that exhibits no Grotthuss shuttling, and a sodium cation. The hydrated excess proton was simulated using a specialized multi-state molecular dynamics method including a proper physical treatment of the proton shuttling and charge defect delocalization. Both AQP1 mutants exhibit a surprising cooperative effect leading to a reduction in the free-energy barrier for proton permeation around the NPA region due to altered water configurations in the SF region, with AQP1-R195S having a higher conductance than AQP1-R195V. The theoretical predictions are experimentally confirmed in wild-type AQP1 and the mutants expressed in Xenopus oocytes. The combined results suggest that the SF domain is a specialized structure that has evolved to impede proton permeation in aquaporins.
authors.affiliation: Department of Chemistry, James Franck Institute, Institute for Biophysical Dynamics, and Computation Institute, University of Chicago, 5735 South Ellis Avenue, Chicago, IL 60637, USA.
authors.forename: Hui
authors.initials: H
authors.lastname: Li
authors.forename: Hanning
authors.initials: H
authors.lastname: Chen
authors.forename: Christina
authors.initials: C
authors.lastname: Steinbronn
authors.forename: Binghua
authors.initials: B
authors.lastname: Wu
authors.forename: Eric
authors.initials: E
authors.lastname: Beitz
authors.forename: Thomas
authors.initials: T
authors.lastname: Zeuthen
authors.forename: Gregory A
authors.initials: GA
authors.lastname: Voth
mesh_terms: D000818:Animals; D051398:Aquaporin 1 / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D002412:Cations / Q000378:metabolism*; D003001:Cloning, Molecular; D019295:Computational Biology; D003198:Computer Simulation; D015870:Gene Expression; D008958:Models, Molecular; D050505:Mutant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D020125:Mutation, Missense; D009865:Oocytes / Q000378:metabolism; D017434:Protein Structure, Tertiary; D011522:Protons; D055672:Static Electricity; D014981:Xenopus
chemical_list: D002412:Cations; D050505:Mutant Proteins; D011522:Protons; D051398:Aquaporin 1