Relationship of membrane sidedness to the effects of the lipophosphoglycanof Leishmania donovani on the fusion of influenza virus

Cells expressing the influenza hemagglutinin protein were fused to planar l ipid bilayers containing the viral receptor G(D1a) at pH 5.0. An amphiphile known to alter membrane propel-ties is lipophosphoglycan (LPG), This glyco conjugate was added from aqueous solution to either the cis or the trans mo nolayer to examine its effects on the fusion process. LPG markedly inhibite d the formation of fusion pens when present in the cis monolayer but LPG in the trans monolayer had no effect on the parameters of pore formation or o n the properties of the pores. The N-terminal segment of the HA2 subunit of the influenza hemagglutinin protein is important for membrane fusion. The effect of LPG on the conformation and membrane insertion of a synthetic 20- amino-acid peptide, corresponding to the influenza fusion peptide, was exam ined at pH 5.0 by attenuated total reflection Fourier transform infrared sp ectroscopy and by the fluorescence properties of the Trp residues of this p eptide. It was found that cis LPG did not prevent insertion of the peptide into the membrane but it did alter the conformation of the membrane-inserte d peptide from alpha-helix to beta-structure. The beta-structure was orient ed along the bilayer normal. The effect of cis LPG on the conformation of t he fusion peptide probably contributes to the observed inhibition of pore f ormation and lipid mixing. In contrast, trans LPG has no effect on the conf ormation or angle of membrane insertion of the peptide, nor does it affect pore formation by HA-expressing cells. The ineffectiveness of trans LPG, de spite it having strong positive curvature-promoting properties, may be a co nsequence of the size of this amphiphile being too large to enter a fusion pore.