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

is presumed to be similar to that of other enveloped viruses. Virion formation makes use of all three forms of the glycoprotein, S (24 kD, which we have described in this work) and the two longer forms $2 (31 kD) and S1 (39 kD) which are produced from three in frame ATGs in the S gene (Ganem and Varmus, 1987) . $2 and S are found in both the virion and the lipoprotein particles while S1 is found only in the virion. An appealing hypothesis is that the virion incorporates the dimeric forms of the glycoprotein (S, S1, $2, and hybrids) before they exit from the ER while the excess S and $2 dimers are transported to an intermediate compartment in which maturation to lipoprotein particles occurs. This hypothesis is supported by the observation that coexpression of the longer S1 form of the HBsAg glycoprotein blocks the secretion of all forms of I-IBsAg (Pershing et al., 1986) . Since the S1 form has been shown to have a retention signal (Kuroki et al., 1989) , formation of oligomers with the other forms of HBsAg would prevent their transport and maturation. The scavenging of the S1 form by budding virions would then allow the other forms to proceed to the intermediate compartment and to lipoprotein particle formation.