Mitotic Spindle Positioning (MISP) is an actin bundler that selectively stabilizes the rootlets of epithelial...

Area of Science:

• Cell Biology
• Biochemistry
• Epithelial Biology

Background:

• Microvilli are essential actin-based cellular protrusions involved in diverse cell functions, particularly in transporting epithelia.
• Existing knowledge identifies villin, fimbrin, and espin as key actin bundlers in microvilli core structure.
• Microvilli biogenesis persists even without these known bundlers, indicating the involvement of unidentified factors.

Purpose of the Study:

• To identify novel actin-binding proteins involved in microvilli biogenesis.
• To elucidate the molecular mechanisms underlying the robustness of microvilli formation.
• To understand the regulation of actin filament organization within microvilli.

Main Methods:

• Protein identification and localization studies within epithelial cells.
• In vitro actin filament bundling assays using purified proteins.
• Co-localization and functional interaction studies involving MISP, fimbrin, and ezrin.

Main Results:

• Mitotic Spindle Positioning (MISP) was identified as a novel actin-binding protein localizing to the microvillus rootlet.
• Purified MISP demonstrates potent actin filament bundling activity in vitro.
• MISP bundling promotes rootlet elongation and recruits fimbrin, enhancing bundle stability.
• Ezrin regulates MISP localization, confining it to the rootlet and preventing distal end bundling.

Conclusions:

• MISP is a novel and essential actin bundler for microvilli biogenesis, particularly at the rootlet.
• The interplay between MISP and ezrin ensures proper actin organization and structural integrity of microvilli.
• These findings reveal new insights into the mechanisms maintaining epithelial apical surface area and the adaptability of microvilli formation.