Calcium control of the intestinal microvillus cytoskeleton: its implications for the regulation of microfilament organizations.

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.