Kerff Frédéric, Amoroso Ana, Herman Raphaël, Sauvage Eric, Petrella Stéphanie, Filée Patrice, Charlier Paulette, Joris Bernard, Tabuchi Akira, Nikolaidis Nikolas, Cosgrove Daniel J
Université de Liège, Centre d'Ingénierie des Protéines, Institut de Chimie B6, 4000 Liège, Belgium.
Proc Natl Acad Sci U S A. 2008 Nov 4;105(44):16876-81. doi: 10.1073/pnas.0809382105. Epub 2008 Oct 29.
We solved the crystal structure of a secreted protein, EXLX1, encoded by the yoaJ gene of Bacillus subtilis. Its structure is remarkably similar to that of plant beta-expansins (group 1 grass pollen allergens), consisting of 2 tightly packed domains (D1, D2) with a potential polysaccharide-binding surface spanning the 2 domains. Domain D1 has a double-psi beta-barrel fold with partial conservation of the catalytic site found in family 45 glycosyl hydrolases and in the MltA family of lytic transglycosylases. Domain D2 has an Ig-like fold similar to group 2/3 grass pollen allergens, with structural features similar to a type A carbohydrate-binding domain. EXLX1 bound to plant cell walls, cellulose, and peptidoglycan, but it lacked lytic activity against a variety of plant cell wall polysaccharides and peptidoglycan. EXLX1 promoted plant cell wall extension similar to, but 10 times weaker than, plant beta-expansins, which synergistically enhanced EXLX1 activity. Deletion of the gene encoding EXLX1 did not affect growth or peptidoglycan composition of B. subtilis in liquid medium, but slowed lysis upon osmotic shock and greatly reduced the ability of the bacterium to colonize maize roots. The presence of EXLX1 homologs in a small but diverse set of plant pathogens further supports a role in plant-bacterial interactions. Because plant expansins have proved difficult to express in active form in heterologous systems, the discovery of a bacterial homolog opens the door for detailed structural studies of expansin function.
我们解析了一种由枯草芽孢杆菌yoaJ基因编码的分泌蛋白EXLX1的晶体结构。其结构与植物β-伸展蛋白（第1组禾本科花粉过敏原）极为相似，由2个紧密堆积的结构域（D1、D2）组成，有一个潜在的跨两个结构域的多糖结合表面。结构域D1具有双ψβ桶状折叠，在45家族糖基水解酶和溶菌转糖基酶的MltA家族中发现的催化位点有部分保守性。结构域D2具有与第2/3组禾本科花粉过敏原相似的免疫球蛋白样折叠，其结构特征类似于A型碳水化合物结合结构域。EXLX1与植物细胞壁、纤维素和肽聚糖结合，但对多种植物细胞壁多糖和肽聚糖缺乏裂解活性。EXLX1促进植物细胞壁伸展，其作用类似于植物β-伸展蛋白，但强度弱10倍，植物β-伸展蛋白可协同增强EXLX1的活性。编码EXLX1的基因缺失不影响枯草芽孢杆菌在液体培养基中的生长或肽聚糖组成，但会减缓渗透休克后的裂解，并大大降低该细菌在玉米根上定殖的能力。在一小部分但种类多样的植物病原体中存在EXLX1同源物，进一步支持了其在植物-细菌相互作用中的作用。由于植物伸展蛋白已被证明难以在异源系统中以活性形式表达，细菌同源物的发现为伸展蛋白功能的详细结构研究打开了大门。
