Hepatitis B surface antigen assembles in a post-ER, pre-Golgi compartment

Previous work has shown that the time course of secretion matches the time course of HBsAg carbonate extractability which reflects particle budding (Simon et al., 1988) . We wanted to compare the rates of these processes to those of dimer and oligomer crosslink formation seen above. Fig. 7 A shows an autoradiograph of immunoprecipitates of SV24 culture media after various chase times. These precipitates did not contain any detectable contaminants and hence we quantitated HBsAg secretion by measuring the incorporated radioactivity from the immunoprecipitated secreted particles (Fig. 7 B) . We and others have shown that HBsAg secreted from SV24 and other transfected cells is particulate, as judged by EM and CsCI: centrifugation (Huovila, A.-P. J., and S. D. Fuller, manuscript in preparation; Dubois et al., 1980; Moriarty et al., 1981; Liu et al., 1982; Laub et al., 1983) . This is consistent with the fact that none of the secreted HBsAg entered the gel without reduction (data not shown), indicating all of it was crosslinked in the particles. The approximate half-time for the particle secretion from the SV24 cells in our chase conditions was 2 h. The rates of secretion for the glycosylated and nonglycosylated forms of HBsAg were compared by quantitation of the relative intensities of the corresponding bands for the nonreduced material and can be seen to be identical (Fig. 7 B) .