Sheu S Y, Lo S J
Graduate Institute of Microbiology and Immunology, National Yang-Ming Medical College, Taipei, Taiwan, China.
J Gen Virol. 1994 Nov;75 ( Pt 11):3031-9. doi: 10.1099/0022-1317-75-11-3031.
In the serum of hepatitis B virus (HBV)-infected patients, two different types of particles, a 42 nm virion and a 22 nm subviral particle, were identified. The envelope of both particles is composed of three proteins, the large (L), middle (M), and major/small (S) surface proteins but the ratio between these components varies in each. The M protein appears in a lesser amount than the S protein in both virion and subviral particles, although it is translated from the same subgenomic RNA, and this is due to its poor initiation context of translation. In addition, only the glycosylated form of M protein is secreted in contrast to both glycosylated and unglycosylated forms of L and S proteins that are secreted. To investigate the biogenesis of M protein, human hepatoma cells transfected with plasmids containing a mutated HBV DNA were used to produce a high amount of M protein. Electron microscopic observation revealed that despite a higher proportion of the M protein being found in the transfected cells, the secreted surface antigen particles possess similar size and density to 22 nm subviral particles. Detailed biochemical analyses showed the following. (1) The unglycosylated M protein was predominantly present in the microsomal fraction but not present in any other subcellular fractions. (2) The M protein formed 22-nm-like particles in the endoplasmic reticulum (ER) and was retained in the post-ER or pre-Golgi regions. (3) In addition to the complex glycosylated form of M protein, a high-mannose form of M protein could be secreted. (4) Normally, no unglycosylated M protein was secreted. However, glycosylation was not essential for M protein secretion since M protein deprived of glycosylation by tunicamycin treatment was detected in the medium. These findings suggest that (i) the M protein was probably translated and co-translocated into the ER and at least one site was glycosylated before leaving the ER resulting in no secretion of unglycosylated M protein, and (ii) the M protein had two secretion pathways, one through the conventional pathway and the other probably directly through the ER.
在乙型肝炎病毒（HBV）感染患者的血清中，鉴定出两种不同类型的颗粒，一种是42纳米的病毒体，另一种是22纳米的亚病毒颗粒。两种颗粒的包膜均由三种蛋白质组成，即大（L）、中（M）和主/小（S）表面蛋白，但这些成分之间的比例在每种颗粒中各不相同。在病毒体和亚病毒颗粒中，M蛋白的含量均低于S蛋白，尽管它是从相同的亚基因组RNA翻译而来的，这是由于其翻译起始环境不佳所致。此外，与分泌的L蛋白和S蛋白的糖基化和非糖基化形式不同，只有糖基化形式的M蛋白被分泌。为了研究M蛋白的生物发生，使用转染了含有突变HBV DNA质粒的人肝癌细胞来大量产生M蛋白。电子显微镜观察显示，尽管在转染细胞中发现M蛋白的比例较高，但分泌的表面抗原颗粒的大小和密度与22纳米亚病毒颗粒相似。详细的生化分析结果如下：（1）未糖基化的M蛋白主要存在于微粒体部分，而不存在于任何其他亚细胞部分。（2）M蛋白在内质网（ER）中形成22纳米样颗粒，并保留在内质网后或高尔基体前区域。（3）除了M蛋白的复杂糖基化形式外，还可以分泌高甘露糖形式的M蛋白。（4）正常情况下，不分泌未糖基化的M蛋白。然而，糖基化对于M蛋白的分泌并非必不可少，因为在培养基中检测到了经衣霉素处理而缺乏糖基化的M蛋白。这些发现表明：（i）M蛋白可能被翻译并共转运到内质网中，并且在离开内质网之前至少有一个位点被糖基化，导致未糖基化的M蛋白不分泌；（ii）M蛋白有两条分泌途径，一条通过常规途径，另一条可能直接通过内质网。
