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

In the complementary biochemical studies we looked at conditions required for the M protein to acquire first, GalNAc and second, the Golgi modifications galactose and sialic acid. Our in vivo labeling data showed that, under conditions that would be expected to block vesicular transport, where ~98 % of ATP was depleted, a significant fraction of the M protein acquired GalNAc but no detectable Golgi modifications were observed. The same result was seen at temperatures between 14 and 15°C, a condition expected to block ER to Golgi transport (Saraste and Kuismanen, 1984) . In both cases, when the block was reversed by either washing out the 2-deoxy-o-glucose and sodium azide or, for the low temperature experiments, by raising the temperature to 37°C the M protein was efficiently converted to the Golgi forms. This argues strongly that the protein had been blocked in a functional pre-Golgi, intermediate form and not in a transportincompetent state resulting from the blocking conditions.