TprK is a key virulence factor of Treponema pallidum subsp. pallidum (T. pallidum) due to its ability to undergo intra-strain antigenic variation through gene conversion. This mechanism can generate millions of tprK gene and protein variants to allow immune evasion and pathogen persistence during infection. In silico structural modeling supports that TprK is an outer membrane β-barrel with porin function and with several surface-exposed loops, seven of which corresponding to the variable regions. No definitive structural of functional data, however, exist for this protein aside from its role in immune evasion. Studies to elucidate TprK biological function as a porin, are hindered by the evidence that TprK is not abundant on T. pallidum outer membrane, and by the fragility of T. pallidum envelope. To gain insight onto TprK structure and possible function as a porin, we used an Escherichia coli - based expression system that yielded highly pure full-length TprK without any intermediate denaturation step, and proceeded to reconstitute it in detergents and lipid nanodiscs. Visualization of TprK in nanodiscs using negative staining electron microscopy supported that TprK is a monomeric porin in an artificial lipid environment mimicking T. pallidum membrane. Our work provided evidence that TprK is a possible porin transporter of T. pallidum, a biological function compatible with its structural models. These results bring us closer to a comprehensive understanding of the function of this important virulence factor in syphilis pathogenesis and T. pallidum biology.

TprK 是梅毒螺旋体亚种的关键毒力因子。苍白球菌( T. pallidum )，因为它能够通过基因转换进行菌株内抗原变异。这种机制可以产生数百万个tprK基因和蛋白质变体，以允许免疫逃避和病原体在感染期间持续存在。在计算机中结构模型支持 TprK 是一个外膜 β-桶，具有孔蛋白功能和几个表面暴露的环，其中七个环对应于可变区。然而，除了其在免疫逃避中的作用外，该蛋白质没有明确的功能结构数据。阐明 TprK 作为孔蛋白的生物学功能的研究受到 TprK 在苍白球外膜上不丰富的证据以及苍白球包膜的脆弱性的阻碍。为了深入了解 TprK 结构和作为孔蛋白的可能功能，我们使用了大肠杆菌- 基于表达系统，无需任何中间变性步骤即可产生高纯度的全长 TprK，并在去污剂和脂质纳米圆盘中进行重组。使用负染色电子显微镜观察纳米圆盘中的 TprK，支持 TprK 是模拟苍白球膜的人工脂质环境中的单体孔蛋白。我们的工作提供的证据表明 TprK 可能是梅毒螺旋体的孔蛋白转运蛋白，一种与其结构模型相容的生物学功能。这些结果使我们更接近于全面了解这一重要毒力因子在梅毒发病机制和梅毒螺旋体生物学中的功能。