Hwang S B, Lai M M
Howard Hughes Medical Institute, University of Southern California School of Medicine, Los Angeles 90033-1054.
J Virol. 1994 May;68(5):2958-64. doi: 10.1128/JVI.68.5.2958-2964.1994.
Hepatitis delta antigen (HDAg) consists of two species, large (LHDAg) and small (SHDAg), which are identical in sequence except that the large form contains 19 extra amino acids at the C terminus. The large form is prenylated on the Cxxx motif. The small form can trans activate HDV RNA replication, while the large form inhibits it. To determine the molecular basis for their differential functions, we examined the effects of prenylation on the conformation and function of HDAg. We show that the presence of prenylates masks a conformational epitope which is present in SHDAg but hidden in wild-type LHDAg; this epitope becomes exposed in all of the nonprenylated mutant LHDAgs. Prenylation also plays a major role in conferring the trans-dominant negative inhibitory activity of LHDAg, since the loss of prenylation in LHDAg reduced its inhibitory activity. The primary amino acids of the C-terminal sequence also contributed to the maintenance of the HDAg protein conformation; a prenylated LHDAg mutant with a five-amino-acid deletion had an exposed C-terminal epitope. By examining LHDAg mutants which have deletions of various extents of C-terminal sequence, with or without the prenylation motif, we have further shown that all of the prenylated mutants have much higher levels of trans-dominant suppressor activities than do the corresponding nonprenylated mutants. Surprisingly, a few nonprenylated LHDAg mutants were able to trans activate HDV RNA replication, while all of the prenylated ones lost this function. These results suggest that isoprenylates cause the masking of a conformational epitope of HDAg and that conformational differences between the large and small HDAgs account for the differences in their trans-activating and trans-dominant inhibitory biological activities.
丁型肝炎抗原（HDAg）由两种类型组成，即大（LHDAg）和小（SHDAg），它们的序列相同，只是大的形式在C末端含有19个额外的氨基酸。大的形式在Cxxx基序上进行异戊二烯化修饰。小的形式可以反式激活HDV RNA复制，而大的形式则抑制它。为了确定它们不同功能的分子基础，我们研究了异戊二烯化对HDAg构象和功能的影响。我们发现，异戊二烯化修饰掩盖了一个构象表位，该表位存在于SHDAg中，但在野生型LHDAg中隐藏；这个表位在所有未异戊二烯化的突变型LHDAg中暴露。异戊二烯化在赋予LHDAg反式显性负抑制活性方面也起主要作用，因为LHDAg中异戊二烯化的缺失降低了其抑制活性。C末端序列的主要氨基酸也有助于维持HDAg蛋白的构象；一个缺失五个氨基酸的异戊二烯化LHDAg突变体有一个暴露的C末端表位。通过研究具有不同程度C末端序列缺失、有或无异戊二烯化基序的LHDAg突变体，我们进一步表明，所有异戊二烯化的突变体比相应的未异戊二烯化的突变体具有更高水平的反式显性抑制活性。令人惊讶的是，一些未异戊二烯化的LHDAg突变体能够反式激活HDV RNA复制，而所有异戊二烯化的突变体都失去了这个功能。这些结果表明，异戊二烯化导致HDAg构象表位的掩盖，并且大、小HDAg之间的构象差异解释了它们反式激活和反式显性抑制生物活性的差异。
