THE LARGE SURFACE PROTEIN OF HEPATITIS-B VIRUS IS RETAINED IN THE YEAST ENDOPLASMIC-RETICULUM AND PROVOKES ITS UNIQUE ENLARGEMENT

The coding sequences for each of the three envelope proteins of hepatitis B virus (HBV), the major (S), middle (M), and large (L) surface proteins, were expressed in Saccharomyces cerevisiae. Analysis by immunoelectron microscopy of thin sections of yeast cells showed that production of L protein but not of M or S protein provoked morphological changes in the yeast endoplasmic reticulum. A large accumulation of membranous structures connected with the perinuclear cysternae and specifically labeled by a monoclonal antibody directed against the amino-terminal (preS1) sequence of the L protein, was observed. The L protein was post-translationally modified by N- and O-linked glycosylation, indicative of its entry into the yeast secretory pathway and by N-myristoylation of its amino-terminal glycine residue. Deletion of this glycine residue resulted in the synthesis of a nonmyristoylated L protein. Proliferation of the endoplasmic reticulum was comparable in cells producing either the myristoylated or nonmyristoylated L protein, indicating that myristoylation alone is not responsible for the induction of the abnormal membrane morphology.