Transporting epithelial cells like those that line the gut build large arrays of actin-supported protrusions called microvilli, which extend from the apical surface into luminal spaces to increase functional surface area. Although critical for maintaining physiological homeostasis, mechanisms controlling the formation of microvilli remain poorly understood. Here, we report that the inverse-bin-amphiphysin-Rvs (I-BAR)-domain-containing protein insulin receptor tyrosine kinase substrate (IRTKS) (also known as BAIAP2L1) promotes the growth of epithelial microvilli. Super-resolution microscopy and live imaging of differentiating epithelial cells revealed that IRTKS localizes to the distal tips of actively growing microvilli via a mechanism that requires its N-terminal I-BAR domain. At microvillar tips, IRTKS promotes elongation through a mechanism involving its C-terminal actin-binding WH2 domain. IRTKS can also drive microvillar elongation using its SH3 domain to recruit the bundling protein EPS8 to microvillar tips. These results provide new insight on mechanisms that control microvillar growth during the differentiation of transporting epithelial cells and help explain why IRTKS is targeted by enteric pathogens that disrupt microvillar structure during infection of the intestinal epithelium. Postema et al. show that IRTKS (BAIAP2L1) localizes to the distal tips of actively growing epithelial microvilli through its N-terminal I-BAR domain. At these sites, IRTKS drives microvillar elongation directly using its C-terminal actin-binding WH2 domain and indirectly using its SH3 domain to recruit the actin regulatory protein EPS8.
Postema, M. M., Grega-Larson, N. E., Neininger, A. C., & Tyska, M. J. (2018). IRTKS (BAIAP2L1) Elongates Epithelial Microvilli Using EPS8-Dependent and Independent Mechanisms. Current Biology, 28(18), 2876-2888.e4. https://doi.org/10.1016/j.cub.2018.07.022