Glenney J R, Bretscher A, Weber K
Proc Natl Acad Sci U S A. 1980 Nov;77(11):6458-62. doi: 10.1073/pnas.77.11.6458.
The microvillus core-filament bundle from intestinal epithelial cells is a highly ordered structure containing actin and four major associated proteins. Two of these, villin and calmodulin, bind calcium ions (Kd approximately 10(-6) M) in the physiologically important range. Because ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid is present throughout the purification and the isolated cores contain levels of calcium substoichiometric to calmodulin, the protein is bound in the structure without calcium saturation. 10-[3-(4-Methyl-1-piperazinyl)propyl]-2-trifluoromethylphenothiazine, a calmodulin-specific drug, removes the protein from the cores without visibly affecting their ultrastructure. Calmodulin-depleted cores rebind exogenously supplied brain calmodulin. Although the core filaments are stable when the calcium level is less than 10(-7) M, they dissassemble when it is greater than 10(-6) M. This appears to be due to the calcium-sensitive allosteric transition of villin from an F-actin bundling protein to an F-actin severing protein. The actions of the two calcium-binding proteins, villin and calmodulin, are discussed in terms of the calcium sensitivity of the filament bundle. We suggest that villin may act as a calcium-sensitive factor regulating microfilament assembly and disassembly and that calmodulin serves as a buffer modulating the free calcium concentration. This hypothesis may explain some aspects of the physiological process of calcium uptake in the intestine and of the effects of calcium fluxes on the submembranous organization of microfilaments in other cells and tissues.
来自肠上皮细胞的微绒毛核心细丝束是一种高度有序的结构，包含肌动蛋白和四种主要相关蛋白。其中两种蛋白，即绒毛蛋白和钙调蛋白，在生理重要范围内结合钙离子（解离常数约为10⁻⁶ M）。由于在整个纯化过程中都存在乙二醇双（β-氨基乙基醚）-N,N,N',N'-四乙酸，且分离出的核心中钙离子的含量低于钙调蛋白的化学计量，因此该蛋白在结构中结合时并未达到钙饱和状态。10-[3-(4-甲基-1-哌嗪基)丙基]-2-三氟甲基吩噻嗪是一种钙调蛋白特异性药物，它能从核心中去除该蛋白，而不会明显影响其超微结构。去除钙调蛋白的核心能重新结合外源供应的脑钙调蛋白。尽管当钙离子水平低于10⁻⁷ M时核心细丝是稳定的，但当钙离子水平高于10⁻⁶ M时它们会解体。这似乎是由于绒毛蛋白从F-肌动蛋白成束蛋白向F-肌动蛋白切断蛋白的钙敏感别构转变所致。根据细丝束的钙敏感性讨论了两种钙结合蛋白，即绒毛蛋白和钙调蛋白的作用。我们认为绒毛蛋白可能作为一种钙敏感因子调节微丝的组装和解聚，而钙调蛋白则作为一种缓冲剂调节游离钙浓度。这一假说可能解释了肠道中钙吸收生理过程的某些方面，以及钙通量对其他细胞和组织中微丝膜下组织的影响。
