Molecular mechanisms of hollow nano-particle formation by a viral envelope protein

点击翻译按钮获取中文摘要

Virus-like hollow nano-particles in a diameter of 50・500nm are formed when envelope protein of human hepatitis B virus (HBV), which is also referred as surface antigen of HBV (HBsAg), is expressed in animal cells and yeast cells. The expressed protein aggregates on the endoplasmic reticulum (ER) membrane first and then buds out into the lumen as particles containing lipid bilayer derived from ER. In this study, we tried to clarify the molecular mechanisms of this nano-particle structure formation by HBsAg.(1) We constructed several deletion mutants of HBsAg and evaluated their abilities of nano-particle formation, to clarify the essential and/or the unnecessary sequences for particle formation. As the results, among 226 amino acid-residues of HBsAg protein, 54 residues in C-terminal hydrophobic region of 71 residues, 50 residues hydrophilic particle-surface hydrophilic region of 57 residues, and 44 residues in inner-particle hydrophilic region of 50 residues could be deleted from HBsAg sequence without significant loss of particle secretion (more than 50% of wild type HBsAg). C-terminal 54 residues and particle-surface 50 residues (total 104 residues) and C-terminal 54 residues and inner particle 44 residues (total 98 residues) could be also deleted simultaneously without significant loss of particle secretion. These results indicated that these delete-capable sequences are unnecessary for particle formation. At the same time, 18-residue sequence at N-terminal of C-terminal hydrophobic region is defined to be the essential sequence for particle formation.2) The 18-residue essential-sequence was further evaluated by introducing several mutations into this site. The results of the mutational analysis suggest that this region form amphipathic helix lying on the interface of lipid bilayer. The amphipathic helix may have important role for two required properties for particle formation: promoting self-aggregation of HBsAg on ER membrane and generating curvature.