Evolutionary and structural aspects of Solanaceae RNases T2

Abstract Plant RNases T2 are involved in several physiological and developmental processes, including inorganic phosphate starvation, senescence, wounding, defense against pathogens, and the self-incompatibility system. Solanaceae RNases form three main clades, one composed exclusively of S-RNases and two that include S-like RNases. We identified several positively selected amino acids located in highly flexible regions of these molecules, mainly close to the B1 and B2 substrate-binding sites in S-like RNases and the hypervariable regions of S-RNases. These differences between S- and S-like RNases in the flexibility of amino acids in substrate-binding regions are essential to understand the RNA-binding process. For example, in the S-like RNase NT, two positively selected amino acid residues (Tyr156 and Asn134) are located at the most flexible sites on the molecular surface. RNase NT is induced in response to tobacco mosaic virus infection; these sites may thus be regions of interaction with pathogen proteins or viral RNA. Differential selective pressures acting on plant ribonucleases have increased amino acid variability and, consequently, structural differences within and among S-like RNases and S-RNases that seem to be essential for these proteins play different functions.

摘要：植物T2型核糖核酸酶（Plant RNases T2）参与多项生理与发育过程，包括无机磷酸盐饥饿、细胞衰老、创伤应答、病原菌防御以及自交不亲和系统。茄科核糖核酸酶（Solanaceae RNases）可分为三个主要进化枝：其一仅由S-核糖核酸酶（S-RNases）构成，另外两个则包含类S核糖核酸酶（S-like RNases）。本研究鉴定到数个位于这些分子高柔性区域的正向选择氨基酸位点，其中多数紧邻类S核糖核酸酶的B1、B2底物结合位点，以及S-核糖核酸酶的高变区域。S-核糖核酸酶与类S核糖核酸酶在底物结合区域氨基酸柔性上的差异，对于理解RNA结合过程至关重要。例如，在类S核糖核酸酶NT（S-like RNase NT）中，两个正向选择氨基酸残基（Tyr156与Asn134）位于该蛋白分子表面柔性最高的位点。核糖核酸酶NT的表达会因烟草花叶病毒（tobacco mosaic virus）侵染而被诱导，因此这些位点可能是与病原菌蛋白或病毒RNA发生相互作用的区域。作用于植物核糖核酸酶的差异化选择压力提升了氨基酸序列的变异度，进而导致类S核糖核酸酶内部、以及类S核糖核酸酶与S-核糖核酸酶之间出现结构差异，而这些差异似乎是这些蛋白行使不同功能的必要基础。