Association of viral particles and viral proteins with membranes in SA11-infected cells.

Electron microscopy after negative staining of SA11-infected cell homogenates revealed that most of the viral particles are associated with membrane-like material. Many of the particles seemed to be fully enveloped in a membrane. This association could also be detected by the observed cosedimentation of viral proteins and cell membranes. Pulse-chase experiments showed that viral glycoproteins rapidly associate with membranes, whereas most of the structural proteins appearing in the soluble fraction immediately after the pulse were slowly chased into the membrane fraction. The membranes could be further fractionated into at least four fractions differing in density and containing a different distribution of viral proteins. Also, the distribution of label into each of these membrane fractions changed after long chase periods. The inhibition of glycosylation with tunicamycin yielded viral particles without an outer layer, but did not affect the described association with membranes. The possible relationship of this finding to the maturation of the virion is discussed.