Cells across all kingdoms of life actively partition molecules between discrete cellular compartments. In Gram-positive bacteria, a thick and highly cross-linked peptidoglycan cell wall separates the bacterial membrane from the extracellular space, imposing a barrier that must be crossed by proteins whose functions require that they be exposed on the bacterial cell surface. Some surface-exposed proteins, such as the Listeria monocytogenes actin nucleation-promoting factor ActA, remain associated with the bacterial membrane yet somehow thread through tens of nanometers of dense, cross-linked cell wall to expose their N-terminus on the outer surface. Here, we show that entropy can drive the translocation of disordered transmembrane proteins through the Gram-positive cell wall. We develop a physical model predicting that the entropic constraint imposed by a thin periplasm is sufficient to drive translocation of an intrinsically disordered protein like ActA across a porous barrier similar to the cell wall. Consistent with this scenario, we demonstrate experimentally that translocation depends on both the dimensions of the cell envelope and the length of the disordered protein, and that translocation is reversible. We also show that disordered regions from eukaryotic nuclear pore complex proteins are capable of entropy-driven translocation through Gram-positive cell walls. These observations suggest that entropic forces alone, rather than chaperones or chemical energy, are sufficient to drive translocation of certain Gram-positive surface proteins for exposure on the outer surface of the cell wall.

中文翻译：

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通过革兰氏阳性细菌细胞壁的熵驱动无序蛋白的移位

跨越所有生命王国的细胞在不连续的细胞区室之间主动分配分子。在革兰氏阳性细菌中，厚而高度交联的肽聚糖细胞壁将细菌膜与细胞外空间分隔开，形成了必须被蛋白质穿过的屏障，这些蛋白质的功能要求它们必须暴露在细菌细胞表面。一些表面暴露的蛋白质，如单核细胞增生李斯特菌肌动蛋白成核促进因子ActA，仍与细菌膜结合，但以某种方式穿过数十纳米的致密交联细胞壁，使其N末端暴露在外表面。在这里，我们表明熵可以驱动无序跨膜蛋白通过革兰氏阳性细胞壁的转运。我们开发了一个物理模型，预测薄薄的周质所施加的熵约束足以驱动内在无序的蛋白（如ActA）跨过类似于细胞壁的多孔屏障转运。与这种情况一致，我们通过实验证明了易位取决于细胞包膜的尺寸和无序蛋白的长度，并且易位是可逆的。我们还显示，从真核细胞核孔复合蛋白的无序区域能够通过革兰氏阳性细胞壁进行熵驱动的转运。这些观察结果表明，单独的熵力而不是分子伴侣或化学能足以驱动某些革兰氏阳性表面蛋白易位，从而暴露于细胞壁的外表面。