Rico M, Bruix M, González C, Monsalve R I, Rodríguez R
Instituto de Estructura de la Materia, CSIC, Madrid, Spain.
Biochemistry. 1996 Dec 10;35(49):15672-82. doi: 10.1021/bi961748q.
Napin BnIb is a representative member of the 2S albumin seed proteins, which consists of two polypeptide chains of 3.8 and 8.4 kDa linked by two disulfide bridges. In this work, a complete assignment of the 1H spectra of napin BnIb has been carried out by two-dimensional NMR sequence-specific methods and its secondary structure determined on the basis of spectral data. A calculation of the tertiary structure has been performed using approximately 500 distance constraints derived from unambiguously assigned NOE cross-correlations and distance geometry methods. The resulting global fold consists of five helices and a C-terminal loop arranged in a right-handed spiral. The folded protein is stabilized by two interchain disulfide bridges and two additional ones between cysteine residues in the large chain. The structure of napin BnIb represents a third example of a new and distinctive folding pattern first described for the hydrophobic protein from soybean and nonspecific lipid transfer proteins from wheat and maize. The presence of an internal cavity is not at all evident, which rules out in principle the napin BnIb as a carrier of lipids. The determined structure is compatible with activities attributed to these proteins such as phospholipid vesicle interaction, allergenicity, and calmodulin antagonism. Given the sequence homology of BnIb with other napins and napin-type 2S albumin seed proteins from different species, it is likely that all these proteins share a common architecture. The determined structure will be crucial to establish structure-function relationships and to explore the mechanisms of folding, processing, and deposition of these proteins. It will also provide a firm basis for a rational use of genetic engineering in order to develop improved transgenic plants.
Napin BnIb是2S白蛋白种子蛋白的典型成员，它由两条分别为3.8 kDa和8.4 kDa的多肽链通过两个二硫键相连组成。在本研究中，通过二维核磁共振序列特异性方法对napin BnIb的1H谱进行了完整归属，并根据光谱数据确定了其二级结构。利用从明确归属的NOE交叉相关和距离几何方法推导得到的约500个距离约束条件进行了三级结构计算。所得的整体折叠结构由五个螺旋和一个C末端环组成，呈右手螺旋排列。折叠后的蛋白质通过两个链间二硫键以及大链中半胱氨酸残基之间的另外两个二硫键得以稳定。napin BnIb的结构代表了一种新的独特折叠模式的第三个例子，这种模式最初是针对大豆中的疏水蛋白以及小麦和玉米中的非特异性脂质转运蛋白描述的。内部腔隙的存在并不明显，这从原则上排除了napin BnIb作为脂质载体的可能性。所确定的结构与这些蛋白质所具有的活性如磷脂囊泡相互作用、致敏性和钙调蛋白拮抗作用是相符的。鉴于BnIb与其他napin以及来自不同物种的napin型2S白蛋白种子蛋白的序列同源性，很可能所有这些蛋白质都具有共同的结构。所确定的结构对于建立结构-功能关系以及探索这些蛋白质的折叠、加工和沉积机制至关重要。它也将为合理利用基因工程以培育改良的转基因植物提供坚实的基础。
