Structure and Dynamics of Extracellular Loops in Human Aquaporin-1 from Solid-State NMR and Molecular Dynamics

Multiple moderate-resolution crystal structures of human aquaporin-1 have provided a foundation for understanding the molecular mechanism of selective water translocation in human cells. To gain insight into the interfacial structure and dynamics of human aquaporin-1 in a lipid environment, we performed nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics simulations. Using magic angle spinning solid-state NMR, we report a near complete resonance assignment of the human aquaporin-1. Chemical shift analysis of the secondary structure identified pronounced deviations from crystallographic structures in extracellular loops A and C, including the cis Y37-P38 bond in loop A, as well as ordering and immobilization of loop C. Site-specific H/D exchange measurements identify a number of protected nitrogen-bearing side chains and backbone amide groups, involved in stabilizing the loops. A combination of molecular dynamics simulations with NMR-derived restraints and filtering based on solvent accessibility allowed for the determination of a structural model of extracellular loops largely consistent with NMR results. The simulations reveal loop stabilizing interactions that alter the extracellular surface of human AQP1, with possible implications for water transport regulation through the channel. Modulation of water permeation may occur as a result of rearrangement of side chains from loop C in the extracellular vestibule of hAQP1, affecting the aromatic arginine selectivity filter.