The use of circular dichroism to study conformational changes induced in Sendai virus envelope glycoproteins. A correlation with the viral fusogenic activity

Fusion of Sendai virus envelopes with erythrocyte membranes or with phosphatidylcholine/cholesterol liposomes requires the presence of the two viral glycoproteins, namely the hemagglutinin/neuraminidase (HN) and the fusion (F) polypeptides. Membrane vesicles bearing only the HN or the F glycoprotein (HN or F vesicles) or a mixture of both do not possess fusogenic activity. These results clearly show that in order to be fusogenic, the two viral envelope glycoproteins must be present within the same membrane, thus indicating their mutual interaction. Circular dichroism studies revealed that the conformation of the viral glycoproteins in reconstituted viral envelopes or in HN-F vesicles (vesicles formed by co-reconstitution of the HN and F glycoproteins) is different from that of the conformation of these glycoproteins in either HN or F vesicles or in a mixture of both. It has been observed that the mean residue ellipticity as measured at 222 nm (θ₂₂₂) of the viral glycoproteins in reconstituted Sendai virus envelopes (RSVE) is lower by about 75% than the value observed for these glycoproteins in isolated HN or F vesicles. Treatment of RSVE or of HN-F vesicles with inhibitors of the viral fusogenic activity such as phenylmethylsulfonyl fluoride, proteolytic enzymes, or incubation at 70°C caused a substantial conformational change in the viral glycoproteins. The θ₂₂₂ of unfusogenic RSVE or unfusogenic HN-F vesicles is very close to that observed for a mixture of HN and F vesicles. It is proposed here that in order to be fusogenic, the viral envelope glycoproteins must possess a certain conformation which exists only when they are present within the same membrane.