Georgelis Nikolaos, Nikolaidis Nikolas, Cosgrove Daniel J
Simplot Plant Sciences, J.R. Simplot Company, Boise, ID, 83706, USA.
Appl Microbiol Biotechnol. 2015 May;99(9):3807-23. doi: 10.1007/s00253-015-6534-0. Epub 2015 Apr 2.
The discovery of microbial expansins emerged from studies of the mechanism of plant cell growth and the molecular basis of plant cell wall extensibility. Expansins are wall-loosening proteins that are universal in the plant kingdom and are also found in a small set of phylogenetically diverse bacteria, fungi, and other organisms, most of which colonize plant surfaces. They loosen plant cell walls without detectable lytic activity. Bacterial expansins have attracted considerable attention recently for their potential use in cellulosic biomass conversion for biofuel production, as a means to disaggregate cellulosic structures by nonlytic means ("amorphogenesis"). Evolutionary analysis indicates that microbial expansins originated by multiple horizontal gene transfers from plants. Crystallographic analysis of BsEXLX1, the expansin from Bacillus subtilis, shows that microbial expansins consist of two tightly packed domains: the N-terminal domain D1 has a double-ψ β-barrel fold similar to glycosyl hydrolase family-45 enzymes but lacks catalytic residues usually required for hydrolysis; the C-terminal domain D2 has a unique β-sandwich fold with three co-linear aromatic residues that bind β-1,4-glucans by hydrophobic interactions. Genetic deletion of expansin in Bacillus and Clavibacter cripples their ability to colonize plant tissues. We assess reports that expansin addition enhances cellulose breakdown by cellulase and compare expansins with distantly related proteins named swollenin, cerato-platanin, and loosenin. We end in a speculative vein about the biological roles of microbial expansins and their potential applications. Advances in this field will be aided by a deeper understanding of how these proteins modify cellulosic structures.
微生物伸展蛋白的发现源于对植物细胞生长机制和植物细胞壁伸展性分子基础的研究。伸展蛋白是一类能使细胞壁松弛的蛋白质，在植物界普遍存在，也在一小部分系统发育多样的细菌、真菌及其他生物中被发现，其中大多数定殖于植物表面。它们能在无明显裂解活性的情况下使植物细胞壁松弛。细菌伸展蛋白因其在纤维素生物质转化用于生物燃料生产方面的潜在用途，即作为一种通过非裂解方式（“无定形化”）分解纤维素结构的手段，最近受到了相当多的关注。进化分析表明，微生物伸展蛋白起源于从植物进行的多次水平基因转移。枯草芽孢杆菌的伸展蛋白BsEXLX1的晶体学分析表明，微生物伸展蛋白由两个紧密堆积的结构域组成：N端结构域D1具有类似于糖基水解酶家族45酶的双ψβ桶折叠，但缺乏水解通常所需的催化残基；C端结构域D2具有独特的β三明治折叠，带有三个共线的芳香族残基，通过疏水相互作用结合β-1,4-葡聚糖。在芽孢杆菌和棒状杆菌中对伸展蛋白进行基因缺失会削弱它们定殖植物组织的能力。我们评估了有关添加伸展蛋白可增强纤维素酶对纤维素分解作用的报道，并将伸展蛋白与远缘相关的蛋白质膨胀素、角质素和松弛素进行了比较。我们以一种推测的方式探讨了微生物伸展蛋白的生物学作用及其潜在应用。对这些蛋白质如何修饰纤维素结构的更深入理解将有助于该领域的进展。
