[Show abstract][Hide abstract] ABSTRACT: It has been supposed that the HA (haemagglutinin) of influenza virus must be recruited to membrane rafts to perform its function in membrane fusion and virus budding. In the present study, we aimed at substantiating this association in living cells by biophysical methods. To this end, we fused the cyan fluorescent protein Cer (Cerulean) to the cytoplasmic tail of HA. Upon expression in CHO (Chinese-hamster ovary) cells HA-Cer was glycosylated and transported to the plasma membrane in a similar manner to authentic HA. We measured FLIM-FRET (Förster resonance energy transfer by fluorescence lifetime imaging microscopy) and showed strong association of HA-Cer with Myr-Pal-YFP (myristoylated and palmitoylated peptide fused to yellow fluorescent protein), an established marker for rafts of the inner leaflet of the plasma membrane. Clustering was significantly reduced when rafts were disintegrated by cholesterol extraction and when the known raft-targeting signals of HA, the palmitoylation sites and amino acids in its transmembrane region, were removed. FRAP (fluorescence recovery after photobleaching) showed that removal of raft-targeting signals moderately increased the mobility of HA in the plasma membrane, indicating that the signals influence access of HA to slowly diffusing rafts. However, Myr-Pal-YFP exhibited a much faster mobility compared with HA-Cer, demonstrating that HA and the raft marker do not diffuse together in a stable raft complex for long periods of time.
[Show abstract][Hide abstract] ABSTRACT: Influenza virus hemagglutinin (HA) has been suggested to be enriched in liquid-ordered lipid domains named rafts, which represent an important step in virus assembly. We employed Förster resonance energy transfer (FRET) via fluorescence lifetime imaging microscopy to study the interaction of the cytoplasmic and transmembrane domain (TMD) of HA with agly co sylphos pha tidyl ino si tol (GPI)-anchored peptide, an established marker for rafts in the exoplasmic leaflet of living mammalian plasma membranes. Cyan fluorescent protein (CFP) was fused to GPI, whereas the HA sequence was tagged with yellow fluorescent protein (YFP) on its exoplasmic site (TMD-HA-YFP), avoiding any interference of fluorescent proteins with the proposed role of the cytoplasmic domain in lateral organization of HA. Constructs were expressed in Chinese hamster ovary cells (CHO-K1) for which cholesterol-sensitive lipid nanodomains and their dimension in the plasma membrane have been described (Sharma, P., Varma, R., Sarasij, R. C., Ira, Gousset, K., Krishnamoorthy, G., Rao, M., and Mayor, S. (2004) Cell 116, 577-589). Upon transfection in CHO-K1 cells, TMD-HA-YFP is partially expressed as a dimer. Only dimers are targeted to the plasma membrane. Clustering of TMD-HA-YFP with GPI-CFP was observed and shown to be reduced upon cholesterol depletion, a treatment known to disrupt rafts. No indication for association of TMD-HA-YFP with GPI-CFP was found when palmitoylation, an important determinant of raft targeting, was suppressed. Clustering of TMD-HA-YFP and GPI-CFP was also observed in purified plasma membrane suspensions by homoFRET. We concluded that the pal mit oy lated TMD-HA alone is sufficient to recruit HA to cholesterol-sensitive nanodomains. The corresponding construct of the spike protein E2 of Semliki Forest virus did not partition preferentially in such domains.