Regulation of Ebola GP conformation and membrane binding by the chemical environment of the late endosome.

Interaction between the Ebola virus envelope glycoprotein (GP) and the endosomal membrane is an essential step during virus entry into the cell. Acidic pH, Ca 2+ , and removal of the glycan cap from GP have been implicated in mediating the GP-membrane interaction. However, the molecular mechanism by which these factors regulate the conformational changes that enable engagement of GP with the target membrane is unknown. Here, we apply fluorescence correlation spectroscopy (FCS) and single-molecule Förster resonance energy transfer (smFRET) to elucidate how the chemical environment of the late endosome promotes GP-membrane interaction, thereby facilitating virus entry. We first investigate the role of anionic phospholipids, phosphatidylserine (PS) and bis(monoacylglycero)phosphate (BMP), which are found in the membrane of the late endosome. We find that these lipids enable robust binding of GP to membranes in a pH- and Ca 2+ -dependent manner. We then identify residues in GP that sense pH and trigger conformational changes that make the fusion loop available for insertion into the membrane. Molecular dynamics (MD) simulations suggest the structural basis for pH-trigger conformational changes. We similarly confirm residues in the fusion loop that mediate GP's interaction with Ca 2+ , which likely promotes local conformational changes in the fusion loop and mediates electrostatic interactions with the anionic phospholipids. Collectively, our results provide a mechanistic understanding of how the environment of the late endosome regulates the timing and efficiency of virus entry.