Shaping the epithelial cell surface with actin bundling proteins

During differentiation, enterocytes build an extensive apical array of microvilli known as the brush border, which serves to amplify the plasma membrane surface area available for nutrient absorption. In addition to serving as the sole site of nutrient uptake, brush border microvilli also provide an anchoring point for the glycocalyx and regulate interactions with luminal microbes. An individual microvillus is simple in structure, consisting of a supporting core of ~25 actin filaments bundled in parallel by villin, fimbrin, and espin. Remarkably, microvilli biogenesis persists in mice lacking all three of these factors, suggesting the existence of unknown bundlers. We identified Mitotic Spindle Positioning (MISP) as an actin binding factor that localizes specifically to the rootlet end of the microvillus. MISP promotes rootlet elongation in cells, and purified MISP exhibits potent filament bundling activity in vitro. MISP-bundled filaments also recruit fimbrin, which further elongates and stabilizes bundles. MISP confinement to the rootlet is enforced by ezrin, which prevents decoration of the membrane-wrapped distal end of the core bundle. These discoveries reveal how enterocytes optimize apical membrane surface area and offer insight on the remarkable robustness of microvilli biogenesis.