Saxton M J
Department of Agronomy and Range Science, University of California, Davis 95616.
Biophys J. 1987 Dec;52(6):989-97. doi: 10.1016/S0006-3495(87)83291-5.
Lateral diffusion of mobile proteins and lipids (tracers) in a membrane is hindered by the presence of proteins (obstacles) in the membrane. If the obstacles are immobile, their effect may be described by percolation theory, which states that the long-range diffusion constant of the tracers goes to zero when the area fraction of obstacles is greater than the percolation threshold. If the obstacles are themselves mobile, the diffusion constant of the tracers depends on the area fraction of obstacles and the relative jump rate of tracers and obstacles. This paper presents Monte Carlo calculations of diffusion constants on square and triangular lattices as a function of the concentration of obstacles and the relative jump rate. The diffusion constant for particles of various sizes is also obtained. Calculated values of the concentration-dependent diffusion constant are compared with observed values for gramicidin and bacteriorhodopsin. The effect of the proteins as inert obstacles is significant, but other factors, such as protein-protein interactions and perturbation of lipid viscosity by proteins, are of comparable importance. Potential applications include the diffusion of proteins at high concentrations (such as rhodopsin in rod outer segments), the modulation of diffusion by release of membrane proteins from cytoskeletal attachment, and the diffusion of mobile redox carriers in mitochondria, chloroplasts, and endoplasmic reticulum.
膜中可移动蛋白质和脂质（示踪剂）的横向扩散会受到膜中蛋白质（障碍物）的阻碍。如果障碍物是固定不动的，其影响可以用逾渗理论来描述，该理论指出，当障碍物的面积分数大于逾渗阈值时，示踪剂的长程扩散常数会趋近于零。如果障碍物本身是可移动的，示踪剂的扩散常数则取决于障碍物的面积分数以及示踪剂与障碍物的相对跳跃速率。本文给出了在正方形和三角形晶格上扩散常数的蒙特卡罗计算结果，该结果是障碍物浓度和相对跳跃速率的函数。同时还得到了不同尺寸粒子的扩散常数。将浓度依赖型扩散常数的计算值与短杆菌肽和细菌视紫红质的观测值进行了比较。蛋白质作为惰性障碍物的影响是显著的，但其他因素，如蛋白质 - 蛋白质相互作用以及蛋白质对脂质粘度的扰动，也具有相当重要的意义。潜在应用包括高浓度蛋白质的扩散（如视杆细胞外段中的视紫红质）、通过膜蛋白从细胞骨架附着释放来调节扩散，以及可移动氧化还原载体在线粒体、叶绿体和内质网中的扩散。
