Structural and functional relationship between the membrane and the cytoskeleton in brush border microvilli

Electron microscopic and biochemical studies have described the organization and composition of microvilli from chicken intestinal brush borders. An actin-based cytoskeleton, composed of a paracrystalline core of bundled microfilaments, maintains the finger-like shape of the membrane through a helical array of membrane-microfilament linkages. Two proteins, fimbrin and villin, are components of the core bundle in situ and can independently bundle the actin filaments in vitro. Structural studies comparing microvillar core bundles with villin bundles and fimbrin bundles suggest that fimbrin, and not villin, is the major actin-filament-bundling protein in the microvillus core. These points, together with the capability of villin to sever actin filaments when activated by Ca2+, raise questions about villin's function in the microvillus. One possible explanation is that villin induces vesiculation of the membrane by disassembling the underlying cytoskeleton.