Much of the molecular motion in the cytoplasm is diffusive, which possibly limits the tempo of processes. We studied the dependence of protein mobility on protein surface properties and ionic strength. We used surface-modified fluorescent proteins (FPs) and determined their translational diffusion coefficients (D) in the cytoplasm of Escherichia coli, Lactococcus lactis and Haloferax volcanii. We find that in E. coli D depends on the net charge and its distribution over the protein, with positive proteins diffusing up to 100-fold slower than negative ones. This effect is weaker in L. lactis and Hfx. volcanii due to electrostatic screening. The decrease in mobility is probably caused by interaction of positive FPs with ribosomes as shown in in vivo diffusion measurements and confirmed in vitro with purified ribosomes. Ribosome surface properties may thus limit the composition of the cytoplasmic proteome. This finding lays bare a paradox in the functioning of prokaryotic (endo)symbionts.

中文翻译：

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核糖体表面特性可能对细胞质蛋白质组的性质施加限制

细胞质中的大部分分子运动是扩散的，这可能限制了过程的速度。我们研究了蛋白质迁移率对蛋白质表面性质和离子强度的依赖性。我们使用表面修饰的荧光蛋白 (FP) 并测定了它们在大肠杆菌、乳酸乳球菌和 Haloferax volcanii 细胞质中的平移扩散系数 (D)。我们发现，在大肠杆菌中，D 取决于净电荷及其在蛋白质上的分布，正蛋白质的扩散速度比负蛋白质慢 100 倍。这种作用在乳酸乳球菌和 Hfx 中较弱。由于静电屏蔽而产生的火山。流动性的降低可能是由阳性 FP 与核糖体的相互作用引起的，如体内扩散测量所示，并在体外用纯化核糖体证实。因此，核糖体表面特性可能限制细胞质蛋白质组的组成。这一发现揭示了原核（内）共生体功能的悖论。