Viruses get an ESCRT

Dev. Cell, published online 28 May 2013; doi:10.1016/j.devcel.2013.04.003

Lysobisphosphatidic acid (LBPA) is a phospholipid abundant in the cargo-containing intraluminal vesicles (ILVs) that are contained within late endosomes and have the characteristic appearance of multivesicular bodies (MVBs). Viruses such as vesicular stomatitis virus (VSV) enter cells via endocytosis into endosomes, where fusion of the viral envelope with the ILV membrane releases the viral RNA into ILVs. To avoid degradative lysosomes, virus components enter the cytoplasm by back-fusion with the MVB membrane, but the details of this mechanism are unclear. Bissig et al. found that the interaction of ALIX with LBPA-containing membranes is enhanced by electrostatic and hydrophobic interactions and is calcium dependent, with Ca²⁺ and LBPA sharing the same binding domain as ALIX, but at distinct sites. The authors applied an algorithm to predict a membrane-interaction site within this domain of ALIX and could also predict the Ca²⁺-binding site; both sites were verified with mutational analysis and competition studies. Biochemical studies, combined with Trp fluorescence, small-angle X-ray scattering and CD experiments, suggest that a conformational change in ALIX, controlled and stabilized by Ca²⁺, leads to a conformational state of ALIX that may insert into the late endosome bilayer during LBPA engagement. The authors mapped domains in ALIX important for binding to Ca²⁺ and LBPA and monitored infection by VSV to conclude that LBPA recruits Ca²⁺-bound ALIX onto late endosomes and that these interactions as well as the ALIX–ESCRT-III interaction are necessary for delivery of viral components to the cytosol during infection.