Correlation of rotational mobility and flexibility of Sendai virus spike glycoproteins with fusion activity

The rotational mobility of Sendai virus glycoprotein spikes was measured by flash-induced transient dichroism of eosin triplet probes. The possible importance of this molecular motion for function was investigated by parallel assays of hemagglutination and fusion with erythrocytes. For mobility measurements, the glycoproteins were labeled on amino groups with eosin-5-isothiocyanate and on the galactose residues of the oligosaccharide chains with eosin-5-thiosemicarbazide. The decay of the absorption anisotropy of both probes, which has a time constant of about 100-200 musec at 37 degrees is attributed to the rotation of the proteins about an axis normal to the plane of the membrane. This motion was inhibited by crosslinking of the spike proteins with glutaraldehyde or by the specific binding of human erythrocyte glycophorin (a virus receptor) to the HN glycoprotein. Low values of the initial anisotropy for both probes indicate the existence of a second, faster motion. This is attributed to segmental motion of the glycoproteins. Segmental motion is inhibited by crosslinking with glutaraldehyde but appears to be little affected by interaction with glycophorin. The temperature dependence of the segmental and rotational motion of the proteins revealed a pronounced increase in mobility in the range of 30-35 degrees which was not paralleled by the lipid motion of the Sendai virus envelope membrane. Since the temperature dependence of virus-induced hemolysis has a similar characteristic, the mobility of glycoproteins appears to be correlated with the fusion activity. The hemagglutination activity, however, is not dependent on the mobility of the glycoprotein spikes.