A research laboratory is investigating the mechanism of action of a novel antiviral compound. In vitro studies demonstrate that the compound effectively inhibits the replication of a specific enveloped RNA virus. Further analysis reveals that the compound does not interfere with viral entry, genome replication, or protein synthesis. However, electron microscopy of infected cells treated with the compound shows an accumulation of viral glycoproteins within the endoplasmic reticulum and Golgi apparatus, with a significant reduction in the release of infectious virions. The MOST likely target of this antiviral compound is: