Maturation of Sindbis virus in insect and mammalian cells

Schematic representation of the morphogenesis of alphaviruses in cultured vertebrate and invertebrate cells. The post translational proteolytic processing of virus structural proteins after their synthesis from the subgenomic 26S messenger RNA, illustrated at the bottom of the figure, is believed to be similar in both hosts. In the vertebrate host, nucleocapsids are assembled in the cell cytoplasm from capsid protein and 42S progeny RNA. The nucleocapsids are matured by envelopment at the plasma membrane by association with regions of the membrane-containing virus glycoproteins. The virus envelope glycoproteins are processed sequentially by host enzymes and transported from their site of synthesis in the rough endoplasmic reticulum through the Golgi membranes to the plasma membrane. The development of virus components in the invertebrate cell is understood only at the level of electron-microscopic morphology. Three profiles of virus maturation have been reported. Some cultured mosquito cells show virus assembly to occur almost exclusively in membrane-limited cytoplasmic "virus factories". Virus is released into the medium by the exocytotic expulsion of this vesicle by fusion of the vesicle membrane with the plasm membrane. In other instances, virus maturation in mosquito cells follows a pathway similar to that observed in vertebrate cells with nucleocapsids assembled in the cell cytoplasm matured at the plasma membrane. The formation of cytoplasmic vesicles containing large numbers of mature virions has also been reported to occur by the maturation of cytoplasmic nucleocapsids by envelopment in the membrane of the vesicle. The release of these virions may also occur by fusion of the vesicle with the cell surface.

Brown, D.T., and L. Condreay (1985). Replication of alphavirus in mosquito cells. In "The Viruses," S. Schlesinger (ed.), pp. 171-207, Academic Press, NY.