Alteration of viral lipid composition by expression of the phospholipid floppase ABCB4 reduces HIV vector infectivity

The ABCB4 protein is a phosphatidyl choline (PC) floppase that mediates transport of PC from the inner to the outer membrane leaflet. This property enabled us to modulate the lipid composition of HIV vectors and study the effects on membrane composition and infection efficiency.Virus generated in the presence of ABCB4 was enriched in PC and cholesterol but contained less sphingomyelin (SM). Viral titers were reduced 5.9 fold. These effects were not observed with an inactive ABCB4 mutant. The presence of the ABC transport inhibitor verapamil abolished the effect of ABCB4 expression on viral titers.The ABCB4 mediated reduction in infectivity was caused by changes in the viral particles and not by components co purified with the virus because virus made in the presence of ABCB4 did not inhibit virus made without ABCB4 in a competition assay.Incorporation of the envelope protein was not affected by the expression of ABCB4. The inhibitory effect of ABCB4 was independent of the viral envelope as the effect was observed with two different envelope proteins.Our data indicate that increasing the PC content of HIV particles reduces infectivity.Because HIV budding takes place at specialized membrane microdomains which are enriched in cholesterol and sphingolipids (rafts), the lipid content of HIV reflects the composition of these membrane domains [1-4]. However, accumulating evidence suggest that retroviral membrane composition is not just a reflection of the producer cells' membrane but that components of the viral membrane play an important part in the viral life cycle.HIV membrane cholesterol has been shown to be important for viral integrity and function, depletion of cholesterol from HIV by incubation with cyclodextrin results in altered morphology and reduced infectivity of the viral particles [2]. The presence of phosphatidylserine (PS) in the viral membrane is essential for infection of monocytes, but not T cells, by HIV[5]. Whether other membrane components of HIV are