How PIP2 Lipids Regulate the Position and Phosphorylation of the Syntaxin N-Terminus

Abstract

Syntaxin, a member of the family of soluble N-ethylmaleimide-sensitive factor attachment protein receptor proteins can bind and regulate plasma membrane ion channels and neurotransmitter transporters (NSS). Studies of such mechanisms for the dopamine transporter (DAT) have established the N-terminal segment of Syntaxin as the site of direct interactions, and have shown the critical role of highly charged PIP2 lipids in regulating Syntaxin-DAT interactions. We used a computational approach that combines mesoscale continuum modeling of protein-membrane interactions with all-atom molecular dynamics (MD) simulations to compare conformational states of Syntaxin in complex with PIP2-enriched and PIP2-depleted membranes. Our mesoscale approach is based on non-linear Poisson-Boltzmann theory of electrostatics and diffusion-like Cahn-Hilliard dynamics that makes possible the quantitative tracking of lipid-type demixing in the membrane due to the interaction with the protein. The calculations with this method identified strong electrostatic interactions of specific sites of Syntaxin with PIP2 lipids that diffused to their vicinity. MD simulations of the resulting system established that as many as five PIP2 lipid molecules can simultaneously bind Syntaxin. The attending segregation of PIP2 lipids appears to have a dramatic effect on the positioning of the Syntaxin N-terminal segment with respect to the membrane/water interface. These results are discussed in the context of the suggested role of PIP2 lipids in regulating Syntaxin-DAT interactions by modulating phosphorylation of Syntaxin at its N-terminus.