Apart from being a conduit for photoassimilate transport in plants, the phloem serves as a pathway for transport of proteins and RNAs from sites of their synthesis to distant plant parts. As demonstrated for mRNAs and small RNAs such as miRNA and siRNA, their phloem transport is largely involved in responses to environmental cues including stresses and pathogen attacks. RNA molecules are believed to be transported in the phloem in the form of complexes with RNA-binding proteins; however, proteins forming such complexes are generally poorly studied. Here, we demonstrate that the Cucurbita maxima phloem serpin-1 (CmPS1), which has been previously described as a functional protease inhibitor capable of long-distance transport via the phloem, is able to bind RNA in vitro. Among different RNAs tested, CmPS1 exhibits a preference for imperfect RNA duplexes and the highest affinity to tRNA. A characteristic complex formed by CmPS1 with tRNA is not observed upon CmPS1 binding to tRNA-like structures of plant viruses. Mutational analysis demonstrates that the CmPS1 N-terminal region is not involved in RNA binding. Since antithrombin-III, the human protease inhibitor of serpin family most closely sequence-related to CmPS1, is found to be unable to bind RNA, one can suggest that, in its evolution, CmPS1 has gained the RNA binding capability as an additional function likely relevant to its specific activities in the plant phloem.

中文翻译：

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南瓜最大韧皮部丝氨酸蛋白酶抑制剂-1 RNA结合特性的检测和体外研究。

韧皮部除了是植物中光同化运输的管道外，还用作蛋白质和RNA从其合成位点到远处植物部分的运输途径。如针对mRNA和miRNA和siRNA等小RNA所证明的，它们的韧皮部转运在很大程度上参与了对环境线索的响应，包括压力和病原体侵袭。据认为，RNA分子以与RNA结合蛋白复合的形式在韧皮部中运输。然而，形成这种复合物的蛋白质通常研究较少。在这里，我们证明了最大的葫芦韧皮丝氨酸丝氨酸蛋白酶抑制剂1（CmPS1），以前已被描述为能够通过韧皮膜长距离运输的功能性蛋白酶抑制剂，能够在体外结合RNA。在测试的不同RNA中，CmPS1表现出对不完善的RNA双链体的偏好，并且对tRNA的亲和力最高。CmPS1与植物病毒的tRNA样结构结合后，未观察到CmPS1与tRNA形成的特征性复合物。突变分析表明，CmPS1 N末端区域不参与RNA结合。由于发现与CmPS1序列最紧密相关的丝氨酸蛋白酶抑制剂（serpin家族）人类蛋白酶抑制剂抗凝血酶III无法结合RNA，有人认为，在其进化过程中，CmPS1获得了RNA结合能力，这是一种可能的附加功能。与它在植物韧皮部中的特定活动有关。突变分析表明，CmPS1 N末端区域不参与RNA结合。由于发现与CmPS1序列最紧密相关的丝氨酸蛋白酶抑制剂（serpin家族）人类蛋白酶抑制剂抗凝血酶III无法结合RNA，有人认为，在其进化过程中，CmPS1获得了RNA结合能力，这是一种可能的附加功能。与它在植物韧皮部中的特定活动有关。突变分析表明CmPS1 N端区域不参与RNA结合。由于发现与CmPS1序列最紧密相关的丝氨酸蛋白酶抑制剂（serpin家族）人类蛋白酶抑制剂抗凝血酶III无法结合RNA，有人认为，在其进化过程中，CmPS1获得了RNA结合能力，这是一种可能的附加功能。与它在植物韧皮部中的特定活动有关。