Lissenberg A, Vrolijk M M, van Vliet A L W, Langereis M A, de Groot-Mijnes J D F, Rottier P J M, de Groot R J
Virology Division, Department of Infectious Diseases and Immunology, Faculty of Veterinary Medicine, Utrecht University, Yalelaan 1, 3584 CL Utrecht, The Netherlands.
J Virol. 2005 Dec;79(24):15054-63. doi: 10.1128/JVI.79.24.15054-15063.2005.
Group 2 coronaviruses encode an accessory envelope glycoprotein species, the hemagglutinin esterase (HE), which possesses sialate-O-acetylesterase activity and which, presumably, promotes virus spread and entry in vivo by facilitating reversible virion attachment to O-acetylated sialic acids. While HE may provide a strong selective advantage during natural infection, many laboratory strains of mouse hepatitis virus (MHV) fail to produce the protein. Apparently, their HE genes were inactivated during cell culture adaptation. For this report, we have studied the molecular basis of this phenomenon. By using targeted RNA recombination, we generated isogenic recombinant MHVs which differ exclusively in their expression of HE and produce either the wild-type protein (HE+), an enzymatically inactive HE protein (HE0), or no HE at all. HE expression or the lack thereof did not lead to gross differences in in vitro growth properties. Yet the expression of HE was rapidly lost during serial cell culture passaging. Competition experiments with mixed infections revealed that this was not due to the enzymatic activity: MHVs expressing HE+ or HE0 propagated with equal efficiencies. During the propagation of recombinant MHV-HE+, two types of spontaneous mutants accumulated. One produced an anchorless HE, while the other had a Gly-to-Trp substitution at the predicted C-terminal residue of the HE signal peptide. Neither mutant incorporated HE into virion particles, suggesting that wild-type HE reduces the in vitro propagation efficiency, either at the assembly stage or at a postassembly level. Our findings demonstrate that the expression of "luxury" proteins may come at a fitness penalty. Apparently, under natural conditions the costs of maintaining HE are outweighed by the benefits.
第二类冠状病毒编码一种辅助包膜糖蛋白，即血凝素酯酶（HE），它具有唾液酸-O-乙酰酯酶活性，据推测，通过促进病毒粒子与O-乙酰化唾液酸的可逆附着，在体内促进病毒传播和进入。虽然HE在自然感染期间可能提供强大的选择优势，但许多小鼠肝炎病毒（MHV）实验室毒株无法产生该蛋白。显然，它们的HE基因在细胞培养适应过程中失活了。在本报告中，我们研究了这一现象的分子基础。通过使用靶向RNA重组，我们产生了同基因重组MHV，它们仅在HE表达上有所不同，分别产生野生型蛋白（HE+）、无酶活性的HE蛋白（HE0）或根本不产生HE。HE的表达与否并未导致体外生长特性的显著差异。然而，在连续细胞培养传代过程中，HE的表达迅速丧失。混合感染的竞争实验表明，这不是由于酶活性：表达HE+或HE0的MHV以相同效率繁殖。在重组MHV-HE+的繁殖过程中，积累了两种自发突变体。一种产生无锚定的HE，另一种在HE信号肽的预测C末端残基处有甘氨酸到色氨酸的取代。两种突变体均未将HE整合到病毒粒子中，这表明野生型HE在组装阶段或组装后水平降低了体外繁殖效率。我们的研究结果表明，“奢侈”蛋白的表达可能会带来适应性代价。显然，在自然条件下，维持HE的成本超过了其益处。
