Molecular Architecture of the Microvillus Cytoskeleton

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Abstract
This study deals with the molecular organization and function of the components of filament core bundles in intestinal epithelial microvilli. The core, isolated in the absence of free Ca2+, contains five major proteins: the actin (Mr 43 000) that makes up the filaments, the villin (Mr 95 000) and fimbrin (Mr 68 000) that cross-link the filaments together, and the 110 000 Mr polypeptide-calmodulin complex that makes up cross-filaments which project laterally from the core and link it to the inner surface of the microvillus membrane. A minor component of the isolated core, an 80 000 Mr polypeptide, has also been isolated, but its function is unknown. In vitro studies have revealed that villin fragments the actin filaments when the free Ca2+ concentration is increased above 10(-6) M and, as a consequence, treatment of microvillus cores with Ca2+ leads to their partial disassembly. The effects of treating isolated microvilli with various agents, including Ca2+, were therefore examined in terms of the cytoskeletal components that were solubilized and the morphological changes that were induced. Treatment of isolated microvilli with concentrations higher than 10(-6) M free Ca2+ resulted in the appearance of regular constrictions of the microvillus membrane and the solubilization of several cytoskeletal components. It is tentatively suggested that this process may occur reversibly in vivo in the normal functioning of the microvillus.