| Background. Hepatitis B virus (HBV) expresses three co-terminal envelope proteins---the large (L), middle (M), and small (S)---which are located on the surface of the virion. The M and S proteins are both secreted as empty "subviral particles," which exceed virions by 1,000--10,000 fold. The S protein serves as the morphogenic factor for virions and subviral particles, while the L protein is required only for virion formation. Results. We found that co-transfecting an envelope-null replication construct with a small dose of the expression construct for L, M, and S proteins reconstituted efficient virion secretion but only 5% of subviral particles. Whereas secretion of subviral particles was inhibited by the L protein in a dose-dependent manner, virion secretion was inhibited by too high or low L/S protein ratio. Consistent with the results of co-transfection experiments, a point mutation at the -3 position of the S gene translation initiation codon reduced HBsAg secretion by 70% but increased virion secretion Surprisingly, ablating M protein expression reduced virion secretion by at least 50% and increased maturity of virion-associated genomes, which nevertheless could not be rescued by M protein provided in trans. Furthermore, M protein stability was dependent on the co-expression of S protein. Conclusion. Our findings suggest that overproduction of subviral particles is not a prerequisite for efficient virion secretion, and that secretion of virions and subviral particles are fully compatible with the recent demonstration of separate secretion pathways adopted by the two types of particles. Whether the huge excess of subviral particles facilitates the establishment of persistent infection by induction of immune tolerance warrants further investigation. |
