Department of Experimental Medicine and Center of Excellence for Biomedical Research, University of Genova, Viale Benedetto XV/1, 16132 Genova, Italy.
J Biol Chem. 2012 Jan 27;287(5):3009-18. doi: 10.1074/jbc.M111.314559. Epub 2011 Dec 8.
Mimivirus is one the largest DNA virus identified so far, infecting several Acanthamoeba species. Analysis of its genome revealed the presence of a nine-gene cluster containing genes potentially involved in glycan formation. All of these genes are co-expressed at late stages of infection, suggesting their role in the formation of the long fibers covering the viral surface. Among them, we identified the L136 gene as a pyridoxal phosphate-dependent sugar aminotransferase. This enzyme was shown to catalyze the formation of UDP-4-amino-4,6-dideoxy-D-glucose (UDP-viosamine) from UDP-4-keto-6-deoxy-D-glucose, a key compound involved also in the biosynthesis of L-rhamnose. This finding further supports the hypothesis that Mimivirus encodes a glycosylation system that is completely independent of the amoebal host. Viosamine, together with rhamnose, (N-acetyl)glucosamine, and glucose, was found as a major component of the viral glycans. Most of the sugars were associated with the fibers, confirming a capsular-like nature of the viral surface. Phylogenetic analysis clearly indicated that L136 was not a recent acquisition from bacteria through horizontal gene transfer, but it was acquired very early during evolution. Implications for the origin of the glycosylation machinery in giant DNA virus are also discussed.
拟菌病毒是迄今为止发现的最大的 DNA 病毒之一，感染了几种棘阿米巴物种。对其基因组的分析显示，存在一个包含潜在参与聚糖形成的基因的九基因簇。所有这些基因在感染的后期都共同表达，这表明它们在形成覆盖病毒表面的长纤维中的作用。在这些基因中，我们鉴定出 L136 基因为吡哆醛磷酸依赖性糖氨基转移酶。该酶被证明能够催化 UDP-4-氨基-4,6-二脱氧-D-葡萄糖（UDP-赤藓糖）从 UDP-4-酮-6-脱氧-D-葡萄糖的形成，这是一种也参与 L-鼠李糖生物合成的关键化合物。这一发现进一步支持了拟菌病毒编码一种完全独立于变形虫宿主的糖基化系统的假设。赤藓糖与鼠李糖、（N-乙酰）葡萄糖胺和葡萄糖一起被发现是病毒聚糖的主要成分。大多数糖与纤维相关联，证实了病毒表面的荚膜样性质。系统发育分析清楚地表明，L136 不是通过水平基因转移从细菌中最近获得的，而是在进化的早期就获得了。还讨论了巨 DNA 病毒中糖基化机制的起源。
