近日，来自美国密苏里州立大学的研究人员阐明了SARS病毒解旋酶NSP13的作用机制。相关研究成果于5月15日在线发表在Plos ONE上。
研究发现，严重急性呼吸道综合征冠状病毒(SARS-CoV)的非结构蛋白13(NSP13)是一种解旋酶，利用能量水解核苷酸，分开双链DNA(dsRNA)或者是具有5′→3′极性的DNA(dsDNA)。
他们发现，解旋的延迟伴随着双链区域核苷酸的长度增加，这表明在解旋过程中存在中间体。为了阐明中间体的本质，他们利用瞬变动力学来分析nsp13的解旋酶活性。结果发现，解旋酶是以一系列不连续的步骤来参与解旋作用，每一步解旋9.3个碱基对(bp)，其催化速率为每秒30步。因此，nsp13的净解旋率约是每秒280bp。
SARS-CoV内RNA依赖的RNA聚合酶(RdRp)是另一种非结构蛋白nsp12。研究发现，它能够提高核苷酸(RNA/RNA或DNA/DNA)解旋的步长，增强nsp13的催化效率。使用口蹄疫病毒RdRp代替nsp12后，nsp13的活性无明显变化，表明这种增强作用对SARS-CoV的nsp12是特异性的。
研究人员Stefan G. Sarafianos表示，nsp13及nsp12能够以一个协调的模式来发挥作用，提高病毒复制的效率。该研究对理解SARS-CoV RNA合成期间nsp13的作用具有重要意义。（生物谷Deepblue编译）
doi: 10.1371/journal.pone.0036521
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Mechanism of Nucleic Acid Unwinding by SARS-CoV Helicase
Adeyemi O. Adedeji, Bruno Marchand, Aartjan J. W. te Velthuis, Eric J. Snijder, Susan Weiss, Robert L. Eoff, Kamalendra Singh, Stefan G. Sarafianos.
The non-structural protein 13 (nsp13) of Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) is a helicase that separates double-stranded RNA (dsRNA) or DNA (dsDNA) with a 5′→3′ polarity, using the energy of nucleotide hydrolysis.We determined the minimal mechanism of helicase function by nsp13. We showed a clear unwinding lag with increasing length of the double-stranded region of the nucleic acid, suggesting the presence of intermediates in the unwinding process.To elucidate the nature of the intermediates we carried out transient kinetic analysis of the nsp13 helicase activity. We demonstrated that the enzyme unwinds nucleic acid in discrete steps of 9.3 base-pairs (bp) each, with a catalytic rate of 30 steps per second.Therefore the net unwinding rate is ~280 base-pairs per second. We also showed that nsp12, the SARS-CoV RNA-dependent RNA polymerase (RdRp), enhances (2-fold) the catalytic efficiency of nsp13 by increasing the step size of nucleic acid (RNA/RNA or DNA/DNA) unwinding.This effect is specific for SARS-CoV nsp12, as no change in nsp13 activity was observed when foot-and-mouth-disease virus RdRp was used in place of nsp12.Our data provide experimental evidence that nsp13 and nsp12 can function in a concerted manner to improve the efficiency of viral replication and enhance our understanding of nsp13 function during SARS-CoV RNA synthesis.
