A C-TERMINAL PRES1 SEQUENCE IS SUFFICIENT TO RETAIN HEPATITIS-B VIRUSL-PROTEIN IN 293 CELLS

Analysis of deletion and/or site-specific mutants of the hepatitis B v irus (HBV) env gene, expressed in human cells, provided clues about th e mechanism that retains the L protein, the largest gene product, in a pre-Golgi compartment. Differences in secretability of the analyzed v ariants suggest that the N-terminal myristic acid and an internal sequ ence within the PreS1 region function as independent retention signals . N-terminal myristic acid alone neither prevented PreS1 + 2 N-linked glycosylation, which signals cotranslational translocation of the doma in, nor strongly inhibited lumenal budding. Thus, myristic acid by its elf acts by arresting secretion of lumenal, soluble Env particles. By contrast, the internal retention determinant, mapping in the C-termina l portion of PreS1, also prevented budding. In addition, the presence of this PreS1 segment correlated with the depression of PreS1 + 2 glyc osylation. This suggests a connection between L retention and the rece ntly described inhibition of PreS1 + 2 cotranslational translocation. A model can be proposed, according to which HBV surface proteins need to cotranslationally translocate their N-terminal moieties in order to assume a transmembrane topology suitable for particulate assembly and secretion. L protein, whose PreS1 + 2 domain undergoes translocation only posttranslationally, would fail to complete the secretion process . To support this model, we show that forced cotranslational transloca tion of the PreS1 + 2 domain (by attachment of an N-terminal processed signal sequence) results in secretion of L protein. (C) 1995 Academic Press, Inc.