Extracellular bacterial cellulose contributes to biofilm stability and to the integrity of the bacterial cell envelope. In Gram-negative bacteria, cellulose is synthesized and secreted by a multi-component cellulose synthase complex. The BcsA subunit synthesizes cellulose and also transports the polymer across the inner membrane. Translocation across the outer membrane occurs through the BcsC porin, which extends into the periplasm via 19 tetra-tricopeptide repeats (TPR). We present the crystal structure of a truncated BcsC, encompassing the last TPR repeat and the complete outer membrane channel domain, revealing a 16-stranded, β barrel pore architecture. The pore is blocked by an extracellular gating loop, while the extended C terminus inserts deeply into the channel and positions a conserved Trp residue near its extracellular exit. The channel is lined with hydrophilic and aromatic residues suggesting a mechanism for facilitated cellulose diffusion based on aromatic stacking and hydrogen bonding.

中文翻译：

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纤维素合酶外膜通道的结构及其与周质 TPR 结构域的关联。

细胞外细菌纤维素有助于生物膜的稳定性和细菌细胞包膜的完整性。在革兰氏阴性细菌中，纤维素由多组分纤维素合酶复合物合成和分泌。BcsA 亚基合成纤维素，并将聚合物转运穿过内膜。跨外膜的易位通过 BcsC 孔蛋白发生，该孔蛋白通过 19 个四肽重复序列 (TPR) 延伸到周质。我们展示了截短的 BcsC 的晶体结构，包含最后一个 TPR 重复序列和完整的外膜通道域，揭示了 16 链、β 桶孔结构。该孔被细胞外门控环堵塞，而延伸的 C 末端深深插入通道中，并将保守的色氨酸残基定位在其细胞外出口附近。该通道内衬有亲水性和芳香族残基，这表明基于芳香族堆积和氢键的促进纤维素扩散的机制。