Banerjee Ankan, Tsai Chi-Lin, Chaudhury Paushali, Tripp Patrick, Arvai Andrew S, Ishida Justin P, Tainer John A, Albers Sonja-Verena
Molecular Biology of Archaea, Max Planck Institute for terrestrial Microbiology, Karl-von-Frisch-Strasse 10, 35043 Marburg, Germany.
Life Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA.
Structure. 2015 May 5;23(5):863-872. doi: 10.1016/j.str.2015.03.001. Epub 2015 Apr 9.
Archaea employ the archaellum, a type IV pilus-like nanomachine, for swimming motility. In the crenarchaeon Sulfolobus acidocaldarius, the archaellum consists of seven proteins: FlaB/X/G/F/H/I/J. FlaF is conserved and essential for archaellum assembly but no FlaF structures exist. Here, we truncated the FlaF N terminus and solved 1.5-Å and 1.65-Å resolution crystal structures of this monotopic membrane protein. Structures revealed an N-terminal α-helix and an eight-strand β-sandwich, immunoglobulin-like fold with striking similarity to S-layer proteins. Crystal structures, X-ray scattering, and mutational analyses suggest dimer assembly is needed for in vivo function. The sole cell envelope component of S. acidocaldarius is a paracrystalline S-layer, and FlaF specifically bound to S-layer protein, suggesting that its interaction domain is located in the pseudoperiplasm with its N-terminal helix in the membrane. From these data, FlaF may act as the previously unknown archaellum stator protein that anchors the rotating archaellum to the archaeal cell envelope.
古菌利用菌毛（一种IV型菌毛样纳米机器）进行游动。在嗜热栖热菌属的嗜酸热硫化叶菌中，菌毛由七种蛋白质组成：FlaB/X/G/F/H/I/J。FlaF保守且对菌毛组装至关重要，但不存在FlaF的结构。在此，我们截短了FlaF的N端，解析了这种单拓扑膜蛋白分辨率为1.5 Å和1.65 Å的晶体结构。结构显示出一个N端α螺旋和一个八链β折叠片层，其免疫球蛋白样折叠与S层蛋白有显著相似性。晶体结构、X射线散射和突变分析表明，体内功能需要二聚体组装。嗜酸热硫化叶菌唯一的细胞包膜成分是准晶体S层，且FlaF特异性结合S层蛋白，这表明其相互作用结构域位于假周质中，其N端螺旋位于膜内。基于这些数据，FlaF可能作为先前未知的菌毛定子蛋白，将旋转的菌毛锚定到古菌细胞包膜上。
