REPLICATION OF CORONAVIRUS MHV-A59 IN SAC- CELLS - DETERMINATION OF THE 1ST SITE OF BUDDING OF PROGENY VIRIONS

Abstract

During infection of [mouse sarcoma] sac- cells by murine coronavirus MHV [murine hepatitis virus] A59 the intracellular sites at which progeny virions bud correlate with the distribution of the viral glycoprotein E1. Budding is first detectable by EM at 6-7 h post infection in small, smooth, perinuclear vesicles and tubules in a region transitional between the rough endoplasmic reticulum and the Golgi apparatus. At later times the rough endoplasmic reticulum becomes the major site of budding and accumulation of progeny virus particles. Indirect immunofluorescence microscopy shows that E1 is confined at 6 h postinfection to the perinuclear region while at later times it also accumulates in the endoplasmic reticulum. At 6 h postinfection the 2nd viral glycoprotein, E2, is distributed throughout the endoplasmic reticulum and is not restricted to the site at which budding begins. Core protein, the 3rd protein in virions, can be detected 2 h before E1 is detectable and budding begins, and at 6 h postinfection it is distributed throughout the cytosol. Evidently, the time and the site at which the maturation of progeny virions occurs is determined by the accumulation of glycoprotein E1 in intracellular membranes. Only rarely do progeny virions bud directly into the cisternae of the Golgi apparatus but at least some already budded virions are transported to the Golgi apparatus where they occur in structures some of which also contain TTPase, a trans Golgi marker.