Characterization of the budding compartment of mouse hepatitis virus: evidence that transport from the RER to the Golgi complex requires only one vesicular transport step

Our localization studies with Helix pomatia showed clearly that, along the biosynthetic pathway the O-linked sugar, Gal-NAc is first detected in the compartment where MHV buds, in agreement with the conclusions of Tooze et al. (1988) . This is supported by earlier data of several authors (Roth, 1984; Pavelka and Ellinger, 1985; Deschuyteneer et al., 1988; Ihida et al., 1991) who showed that this lectin did not label the rough ER but labeled membrane structures on the cis side of the Golgi complex. In rat liver this lectin labels neither the rough ER nor the bulk of the smooth ER; the labeling is restricted to poorly defined membrane elements close to, and including the Golgi complex (results not shown). These data indicate that the acquisition of GalNAc first occurs in the pre-Golgi budding compartment, a finding that has important implications for our biochemical studies. Collectively, our morphological studies argue that MHV both buds and acquires GalNAc in the so called intermediate compartment that is continuous with the rough ER.