Membrane changes associated with assembly of visna virus

Abstract

Abstract The maturation of visna virus at the surface membranes of sheep choroid plexus cells has been studied by freeze-fracture, deep-etching, surface replication, and scanning EM techniques. The results were integrated with data obtained from thin sections. The first changes detectable in the membranes of infected cells consist of flat regions which lack intramembrane particles and show clusters of globular units on corresponding areas of the surface. Protrusion of the cell membrane, characteristic of the budding process, appears to be initiated after a capsid attaches to these modified membrane regions. During the growth of the viral bud, the areas lacking intramembrane particles, the crescent capsids, and the surface covered with globular units increase in size simultaneously. The surface units become more widely spaced as if they were anchored to the bending capsid. At the end of this development, viruses bud off singly or in rows from the soma and long processes of the infected cells. The capsid detaches from the envelope and forms a central core, the surface units become larger and more closely apposed, and free viruses become smaller than viral buds. Thus, visna virus formation requires insertion and growth of surface units probably representing viral-coded proteins, reorganization of host cell membrane proteins under these units, and, initially, attachment of a coiled capsid to the modified membrane.