Microvilli EM

Electron micrographs of microvilli in cross-section reveal a dense array of actin filaments that provide structural support to the microvilli. How are the closely-packed, parallel array of actin filaments generated in microvilli? Contrast the appearance of actin filaments in electron micrographs with microtubules on the next slide.

Answer: The protein villin crosslinks neighboring actin filaments into bundles. Villin contains two actin-binding domains separated by a short sequence. The closeness of the actin-binding domains allows villin to tightly pack actin filaments within microvilli.