The rubella virus E1 glycoprotein is arrested in a novel post-ER, pre- Golgi compartment

Recently it has been shown that unassembled subunits of multimeric protein complexes expressed in transfected cells are rapidly degraded in non-lysosomal, pre-Golgi compartments (I, 7, 3 I). Biosynthetic labeling experiments with extended chase periods were used to determine the stability of El in its pre-Golgi site of accumulation. Subconfluent monolayers of CHOEI cells were pulse labeled for 60 min with [3sS]cysteine and chased for various time periods. El was immunoprecipitated and subjected to SDS-PAGE and fluorography (Fig. II) . Bands were quantitated by scanning densitometry and plotted as a function of time (not shown). The half-life of El was determined to be '~6 h which is comparable to that of unassembled subunits of many other proteins which typically ranges from 6-10 h (22) . A putative 14-kD degradation product accumulated during increased chase periods (Fig. II, arrowhead) . When pulse-chase experiments were carried out in the presence of chloroquine, the drug had very little if any effect on the rate of El proteolysis (not shown) rendering it unlikely that El is degraded in an acidic compartment such as lysosomes or endosomes. Double immunofluorescence experiments indicated that the tubular networks did not contain the lysosome markers Igpl20 (30) and cathepsin D, or the endosome marker, mannose-6-phosphate receptor (not shown). Similarly, El was not detected in these endocytic structures. These results suggest that E1 may undergo degradation at its site of accumulation.