ASSEMBLY OF VACCINIA VIRUS - ROLE OF THE INTERMEDIATE COMPARTMENT BETWEEN THE ENDOPLASMIC-RETICULUM AND THE GOLGI STACKS

Vaccinia virus, the prototype of the Poxviridae, is a large DNA virus which replicates in the cytoplasm of the host cell. The assembly pathw ay of vaccinia virus displays several unique features, such as the pro duction of two structurally distinct, infectious forms. One of these, termed intracellular naked virus (INV), remains cell associated while the other, termed extracellular enveloped virus (EEV), is released fro m the cell. In addition, it has long been believed that INVs acquire t heir lipid envelopes by a unique example of de novo membrane biogenesi s. To examine the structure and assembly of vaccinia virus we have use d immunoelectron microscopy using antibodies to proteins of different subcellular compartments as well as a phospholipid analysis of purifie d INV and EEV. Our data are not consistent with the de novo model of v iral membrane synthesis but rather argue that the vaccinia virus DNA b ecomes enwrapped by a membrane cisterna derived from the intermediate compartment between the ER and the Golgi stacks, thus acquiring two me mbranes in one step. Phospholipid analysis of purified INV supports it s derivation from an early biosynthetic compartment. This unique assem bly process is repeated once more when the INV becomes enwrapped by an additional membrane cisterna, in agreement with earlier reports. The available data suggest that after fusion between the outer envelope an d the plasma membrane, mature EEV is released from the cell.