Rotational Mobility of Sendai Virus Glycoproteins in Membranes of Fused Human Erythrocytes and in the Envelopes of Cell-bound Virions

The rotational mobility of Sendai virus envelope glycoproteins (F, the fusion protein, and HN, the hemagglutinin/neuraminidase) was determined by using erythrosin (ER)-labeled monovalent Fab' antibody fragments directed specifically against either F or HN. By use of time-resolved phosphorescence anisotropy, the rotational mobility of Er-Fab'-viral glycoprotein complexes was studied both in the envelopes of unfused virions bound to erythrocyte ghosts and in the target cell membrane after fusion had occurred. The rotational correlation times (phi) of Er-Fab'-labeled F and HN were rather similar in the envelopes of bound unfused virions, but highly different in membranes of fused cells. The different phi values indicate that F and HN diffuse separately in the target cell membrane and for the major part are not complexed together. The temperature dependence of the phi values of the Er-Fab'-viral glycoprotein complexes revealed a breakpoint at 22 degrees C for the F protein both in bound virions and in the membranes of fused cells, and for the HN proteins in the envelopes of bound virions. In all these cases, the phi values increased between 4 and 22 degrees C, demonstrating a reduction in the rate of rotational diffusion. Further elevation of the temperature reversed the direction of the change in phi. This phenomenon may reflect a temperature-dependent microaggregation of F and HN saturating at ca. 22 degrees C and presumably related to the fusion mechanism since the breakpoint temperature correlates closely with the threshold temperature for virus-cell and cell-cell fusion.(ABSTRACT TRUNCATED AT 250 WORDS)