title: Model of aquaporin-4 supramolecular assembly in orthogonal arrays based on heterotetrameric association of M1-M23 isoforms.
abstract: Tetramers of aquaporin-4 (AQP4) water channels form supramolecular assemblies in cell membranes called orthogonal arrays of particles (OAPs). We previously reported evidence that a short (M23) AQP4 isoform produced by alternative splicing forms OAPs by an intermolecular N-terminus interaction, whereas the full-length (M1) AQP4 isoform does not by itself form OAPs but can coassemble with M23 in OAPs as heterotetramers. Here, we developed a model to predict number distributions of OAP size, shape, and composition as a function M23:M1 molar ratio. Model specifications included: random tetrameric assembly of M1 with M23; intertetramer associations between M23 and M23, but not between M1 and M23 or M1; and a free energy constraint limiting OAP size. Model predictions were tested by total internal reflection fluorescence microscopy of AQP4-green-fluorescent protein chimeras and native gel electrophoresis of cells expressing different M23:M1 ratios. Experimentally validated model predictions included: 1), greatly increased OAP size with increasing M23:M1 ratio; 2), marked heterogeneity in OAP size at fixed M23:M1, with increased M23 fraction in larger OAPs; and 3), preferential M1 localization at the periphery of OAPs. The model was also applied to test predictions about binding to AQP4 OAPs of a pathogenic AQP4 autoantibody found in the neuroinflammatory demyelinating disease neuromyelitis optica. Our model of AQP4 OAPs links a molecular-level interaction of AQP4 with its supramolecular assembly in cell membranes.
authors.affiliation: Department of Medicine, University of California, San Francisco, California, USA.
authors.forename: Byung-Ju
authors.initials: BJ
authors.lastname: Jin
authors.forename: Andrea
authors.initials: A
authors.lastname: Rossi
authors.forename: A S
authors.initials: AS
authors.lastname: Verkman
mesh_terms: D051401:Aquaporin 4 / Q000737:chemistry / Q000378:metabolism*; D002460:Cell Line; D006801:Humans; D007074:Immunoglobulin G / Q000378:metabolism; D008958:Models, Molecular; D009471:Neuromyelitis Optica / Q000276:immunology; D011485:Protein Binding; D020033:Protein Isoforms / Q000737:chemistry / Q000378:metabolism; D055503:Protein Multimerization; D013816:Thermodynamics
chemical_list: D051401:Aquaporin 4; D007074:Immunoglobulin G; D020033:Protein Isoforms