HOMOLOGY MODELING AND MOLECULAR DYNAMICS SIMULATIONS TO STUDY HUMAN AQP4 ISOFORMS

CASAL, JUAN J.; CAPURRO, CLAUDIA; MOLINA PONCE, T; NETTI, VANINA; COCCA, AZUL

Congreso; REUNIÓN DE SOCIEDADES DE BIOCIENCIAS 2020; 2020

Aquaporin-4 (AQP4) is expressed at the plasma membrane as 2 isoforms, AQP4-M1 of 323 amino acids (aa) and AQP4-M23 of 301 aa. Recently, a new AQP4 isoform with a 29 aa C-terminal (Ct) extension (AQP4-Ex) generated by translational readthrough was described. Crystalized AQP4-M1 (32-254 aa) lacks N-terminal (Nt) and Ct ends. Our aim was to model AQP4-M1 and AQP4-Ex by homology modeling to study and compare their properties by molecular dynamics (MD) simulations. Human AQP4 sequences were obtained from UniProt (entry P55087). CBS-DTU tools were used for post-translational modification analysis. AQP4 crystalized structure was obtained from the Protein Data Bank (entry 3GD8) and each Nt and Ct ends were modeled with PEP-FOLD 3 (RPBS Web Portal). Peptides were linked to the 3GD8 AQP4 model to build AQP4-M1 and AQP4-Ex homotetramers by UCSF Chimera software. A 10 ns MD simulation was run in GROMACS 2019 for both isoforms embedded in a bilayer of lipid POPC molecules and solvated with TIP3P as a solvent model. AQP4-Ex (352 aa) had a 100% identity with AQP4-M1. Ct of AQP4-Ex had only two Serine residues (331 and 335) with high score for phosphorylation motif prediction. Homology modeling of AQP4-Ex showed that the extended Ct is a random coil. MD simulations evidenced that AQP4-Ex has a larger mean square displacement and radius of gyration as com pared to AQP4-M1, indicating that AQP4-Ex would be less compact and stable. The distance from His201 to Arg216, representative of the selectivity filter (AQP4-M1: 4.32 ± 0.03 nm vs. AQP4-Ex: 4.27 ± 0.03 nm, n=4, ns), showed that water permeability of these isoforms should be similar. Bioinformatics tools allowed us to model both full-length AQP4 isoforms for the first time. MD simulations of AQP4-Ex provide valuable insights into its water permeation mechanism, which agree with recent experimental observations. This is a promising starting point for performing MD simulations to elucidate the function of this novel extended isoform.