Alanyl-tRNA synthetase (AlaRS)是細胞內必要的轉譯酵素之一，它的主要功能是將丙胺酸(alanine)接到相對應的tRNAAla上。AlaRS藉由辨識tRNAAla acceptor stem上的G3:U70鹼基配對來維持其tRNAAla辨識的專一性，這種辨認方式存在幾乎所有物種中。AlaRS的結構含有四個功能區域(包含催化、tRNA辨識、校正、及C-Ala)，而且在演化上一直保持這四個功能區域，因此不論是原核或真核生物的AlaRS都是由四個功能區域組成。然而，不同於胺基端的其他三個功能區域，他們在序列及功能上都非常保守，C端區域 (C-Ala)在序列和功能上有著顯著差異。例如，大腸桿菌C-Ala參與了tRNA結合，並參與AlaRS二聚體的形成，但人類細胞質C-Ala卻不參與tRNA結合，而且人類AlaRS是以單體的形式存在。更有趣的是，人類C-Ala雖然失去結合tRNA的能力，卻獲得DNA結合能力。在本論文中，我們發現線蟲C. elegans細胞質C-Ala同時具有tRNA和DNA的結合能力，而且此 C-Ala 能夠結合許多不同種類的tRNA，但是明顯地偏好tRNAAla。我們進一步的實驗發現線蟲C-Ala 可以專一性地辨識tRNAAla D-loop上的G18。值得注意的是：這二個鹼基在演化上非常保守，且參與了tRNA L構型的形成，因此這二個鹼基也廣泛存在非tRNAAla的D-loop上。雖然線蟲粒線體AlaRS的C-Ala比其細胞質AlaRS的C-Ala 短小許多(大約只有四分之一大小)，卻也可以同時與DNA及其同源tRNAAla D-loop結合。這項研究解釋了C-Ala如何透過與tRNAAla的D-loop結合，促進 AlaRS 的tRNAAla結合能力及胺醯化活性。;Alanyl-tRNA synthetase (AlaRS) is the enzyme responsible for charging alanine to its cognate tRNAAla. AlaRS is prominent for its ability to specifically recognize tRNAAla through a wobble base pair in the acceptor stem, G3:U70, which is highly conserved in nearly all organisms. AlaRS retains a highly-conserved four-domain structure. Nevertheless, its C-terminal domain (C-Ala) is significantly diverged in sequence and function. For example, E. coli C-Ala mediates tRNA binding and dimerization, while human cytoplasmic C-Ala poorly binds tRNA and forms a monomer. Instead, human C-Ala retains a strong DNA-binding activity. We showed herein that the nematode C. elegans cytoplasmic C-Ala robustly binds both tRNA and DNA. Despite the fact that this C-Ala can bind many different tRNAs with appreciable affinities, with a distinct preference to tRNAAla. As it turns out, Ce-C-Alac specifically recognized the conserved invariant base G18 in the D-loop of tRNAAla through a highly conserved lysine residue, K934. While its mitochondrial counterpart is only one fourth the size of a regular C-Ala domain, it can also bind DNA and its cognate tRNAAla. This study underscores the molecular mechanism of how C-Ala targets AlaRS to the elbow-containing tRNAAla and facilitates its aminoacylation.
