Analysis of SARS-CoV E protein ion channel activity by tuning the protein and lipid charge

Abstract

A partial characterization of the ion channels formed by the SARS coronavirus (CoV) envelope (E) protein was previously reported (C. Verdiá-Báguena et al., 2012 [12]). Here, we provide new significant insights on the involvement of lipids in the structure and function of the CoV E protein channel on the basis of three se- ries of experiments. First, reversal potential measurements over a wide range of pH allow the dissection of the contributions to channel selectivity coming from ionizable residues of the protein transmembrane do- main and also from the negatively charged groups of diphytanoyl phosphatidylserine (DPhPS) lipid. The cor- responding effective pKas are consistent with the model pKas of the acidic residue candidates for titration. Second, the change of channel conductance with salt concentration reveals two distinct regimes (Donnan-controlled electrodiffusion and bulk-like electrodiffusion) fully compatible with the outcomes of selectivity experiments. Third, by measuring channel conductance in mixtures of neutral diphytanoyl phos- phatidylcholine (DPhPC) lipids and negatively charged DPhPS lipids in low and high salt concentrations we conclude that the protein–lipid conformation in the channel is likely the same in charged and neutral lipids. Overall, the whole set of experiments supports the proteolipidic structure of SARS-CoV E channels and ex- plains the large difference in channel conductance observed between neutral and charged membranes.