Mak Amanda Nga-Sze, Wong Yuen-Ting, An Young-Jun, Cha Sun-Shin, Sze Kong-Hung, Au Shannon Wing-Ngor, Wong Kam-Bo, Shaw Pang-Chui
Department of Biochemistry, Centre for Protein Science and Crystallography, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, China.
Nucleic Acids Res. 2007;35(18):6259-67. doi: 10.1093/nar/gkm687. Epub 2007 Sep 13.
Maize ribosome-inactivating protein is classified as a class III or an atypical RNA N-glycosidase. It is synthesized as an inactive precursor with a 25-amino acid internal inactivation region, which is removed in the active form. As the first structural example of this class of proteins, crystals of the precursor and the active form were diffracted to 2.4 and 2.5 A, respectively. The two proteins are similar, with main chain root mean square deviation (RMSD) of 0.519. In the precursor, the inactivation region is found on the protein surface and consists of a flexible loop followed by a long alpha-helix. This region diminished both the interaction with ribosome and cytotoxicity, but not cellular uptake. Like bacterial ribosome-inactivating proteins, maize ribosome-inactivating protein does not have a back-up glutamate in the active site, which helps the protein to retain some activity if the catalytic glutamate is mutated. The structure reveals that the active site is too small to accommodate two glutamate residues. Our structure suggests that maize ribosome-inactivating protein may represent an intermediate product in the evolution of ribosome-inactivating proteins.
玉米核糖体失活蛋白被归类为III类或非典型RNA N-糖苷酶。它作为一种无活性的前体被合成，带有一个25个氨基酸的内部失活区域，该区域在活性形式中被去除。作为这类蛋白质的首个结构实例，前体和活性形式的晶体分别衍射到2.4 Å和2.5 Å。这两种蛋白质相似，主链均方根偏差（RMSD）为0.519。在前体中，失活区域位于蛋白质表面，由一个柔性环和一个长α螺旋组成。该区域减少了与核糖体的相互作用和细胞毒性，但不影响细胞摄取。与细菌核糖体失活蛋白一样，如果催化性谷氨酸发生突变，则玉米核糖体失活蛋白在活性位点没有备用谷氨酸，这有助于该蛋白保留一些活性。该结构表明，活性位点太小，无法容纳两个谷氨酸残基。我们的结构表明，玉米核糖体失活蛋白可能代表核糖体失活蛋白进化过程中的一种中间产物。
