Deletions in the hepatitis B virus small envelope protein: effect on assembly and secretion of surface antigen particles.

The small envelope S protein of hepatitis B virus carrying the surface antigen has the unique property of mobilizing cellular lipids into empty envelope particles which are secreted from mammalian cells. We studied the biogenesis of such particles using site-directed mutagenesis. In this study, we describe the effect of deletions in the N-terminal hydrophobic and hydrophilic domains of the S protein. Whereas short overlapping deletions of hydrophilic sequences flanking the first hydrophobic domain were tolerated, larger deletions of the same sequences were not. Conversely, the hydrophilic region preceding the second hydrophobic domain was not permissive for even short deletions. Deletion of part or all of the first hydrophobic domain also completely blocked secretion, confirming that the entire apolar region serves an essential function. Most of the secretion-defective deletion mutants still entered the secretory pathway and translocated at least the second hydrophilic domain across the membrane of the endoplasmic reticulum. These mutants appeared to remain arrested in a membrane-associated configuration in the endoplasmic reticulum or the cis-Golgi compartment but preserved their capacity for oligomerization with the wild-type S protein. While secretion of wild-type S protein was specifically blocked by the formation of intracellularly retained mixed envelope aggregates, secretion of an unrelated protein (interleukin 9) was completely unaffected.