Hepatitis B virus assembly is sensitive to changes in the cytosolic S loopof the envelope proteins

Among the three related L, M, and S envelope proteins of the hepatitis a vi rus (HBV), the L and S polypeptides are required for virion production. Whe reas the pivotal function of the pre-S region of L in nucleocapsid envelopm ent has been established, the contribution of its S domain and the S protei n is less clear. In this study, we evaluated the role of the cytosolic S lo op, common to L and S, in HBV assembly by performing mutagenesis experiment s. To distinguish between the effect of the mutations on either envelope or virion formation, we investigated the ability of the mutants to assemble i nto secretable subviral empty envelopes and to replace the wild-type protei ns in virion maturation, respectively. Virion production was found to be bl ocked by each of the secretion-competent deletion and substitution mutants S Delta 35-39, S Delta 40-46, S Delta 50-56, and S sigma 58-59, while an in sertion within the loop is tolerated. Surprisingly, single mutations of the arginines terminating the loop had an opposite effect: while a conservativ e exchange of Arg-73 still allowed virion formation, the same mutation of A rg-79 did not. The critical sequences and/or structural requirements of the cytosolic S loop involved in nucleocapsid envelopment primarily act in the S background. These findings can be related to a modal for a synergistical function of both L and S proteins in HBV morphogenesis. (C) 2000 Academic Press.