Preferential electrostatic interactions of phosphatidic acid with arginines.

Phosphatidic acid (PA) is an anionic lipid that preferentially interacts with proteins in a diverse set of cellular processes such as transport, apoptosis, and neurotransmission. One such interaction is that of the PA lipids with the proteins of voltage-sensitive ion channels. In comparison to several other similarly charged anionic lipids, PA lipids exhibit much stronger interactions. Intrigued and motivated by this finding, we sought out to gain deeper understanding into the electrostatic interactions of anionic lipids with charged proteins. Using the voltage sensor domain (VSD) of the KvAP channel as a model system, we performed long-timescale atomistic simulations to analyze the interactions of POPA, POPG, and POPI lipids with arginines (ARGs). Our simulations reveal two mechanisms. First, POPA is able to interact not only with surface ARGs but is able to snorkel and interact with a buried arginine. POPG and POPI lipids on the other hand show weak interactions even with both the surface and buried ARGs. Second, deprotonated POPA with -2 charge is able to break the salt-bridge connection between VSD protein segments and establish its own electrostatic bond with the ARG. Based on these findings, we propose a headgroup size hypothesis for preferential solvation of proteins by charged lipids. These findings may be valuable in understanding how PA lipids could be modulating kinematics of transmembrane proteins in cellular membranes.

Computational design of N-linked glycans for high throughput epitope profiling.

Efficient identification of epitopes is crucial for drug discovery and design as it enables the selection of optimal epitopes, expansion of lead antibody diversity, and verification of binding interface. Although high-resolution low throughput methods like x-ray crystallography can determine epitopes or protein-protein interactions accurately, they are time-consuming and can only be applied to a limited number of complexes. To overcome these limitations, we have developed a rapid computational method that incorporates N-linked glycans to mask epitopes or protein interaction surfaces, thereby providing a mapping of these regions. Using human coagulation factor IXa (fIXa) as a model system, we computationally screened 158 positions and expressed 98 variants to test experimentally for epitope mapping. We were able to delineate epitopes rapidly and reliably through the insertion of N-linked glycans that efficiently disrupted binding in a site-selective manner. To validate the efficacy of our method, we conducted ELISA experiments and high-throughput yeast surface display assays. Furthermore, x-ray crystallography was employed to verify the results, thereby recapitulating through the method of N-linked glycans a coarse-grained mapping of the epitope.

A lateral electric field inhibits gel-to-fluid transition in lipid bilayers.

We report evidence of lateral electric field-induced changes in the phase transition temperatures of lipid bilayers. Our atomic scale molecular dynamics simulations show that a lateral electric field increases the melting temperatures of DPPC, POPC and POPE bilayers. Remarkably, these shifts in the melting temperatures are only induced by lateral electric fields, and not normal electric fields. This mechanism could provide new mechanistic insights into lipid-lipid and lipid-protein interactions in the presence of endogenous and exogenous electric fields.