Effect of Mutations on Transport by the Sodium/Iodide Symporter (NIS)

Abstract

The Na+/I- symporter (NIS) mediates iodide transport into the thyroid and other tissues, such as salivary glands, stomach, lactating breast, and small intestine. To understand the observed effects of single amino acid substitutions in the mechanism of transport by NIS, wild type NIS and NIS mutated at selected positions were studied using computational methods. Normal modes analysis with the Anisotropic Network Model (ANM) was used to explore the changes in global movements between NIS and the mutants in the presence or absence of the ions they transport (Na+, I-, and ClO4-). ANM has been shown to be a useful computational tool for predicting the dynamics of membrane proteins in many applications. The lowest normal modes generated by the ANM provide valuable insight into the global dynamics of biomolecules that are relevant to their function. We employed a modified version of ANM in which closeness of side chains are also used to assign alpha-carbons connections, allowing us to identify differences due to mutations and the influence of ions on these global movements, thus enabling a first approximation to the influence of mutations on the transport mechanism of NIS.To address questions relating to the coordination of the transported ions in the wild type and in mutants, we carried out molecular dynamics simulations in a lipid bilayer using an implicit solvent model with GBMV (Generalized Born Molecular Volume) in both wild-type NIS and NIS mutants and their ions. A homology model of NIS based on the structure of the bacterial homologue vSGLT (Vibrio parahaemolyticus Na+/galactose transporter) was used in both the normal modes analysis and the molecular dynamics simulations.