Mechanisms of BAR Protein Function in Brush Border Assembly

Abstract

The intestinal brush border lines the apical surface of enterocytes and is composed of thousands of actin-supported protrusions called microvilli, which extend into the intestinal lumen. Microvilli function to increase intestinal surface area and are critical for the homeostasis of transporting epithelia, yet mechanisms that control the assembly and morphology of these protrusions remain poorly understood. Here we report that two BAR domain-containing proteins, insulin receptor tyrosine kinase substrate (IRTKS) and protein kinase C and casein kinase substrate in neurons 2 (PACSIN2), are necessary components of brush border assembly. Through super-resolution imaging, IRTKS was found to localize to the distal tips of microvilli where it promotes the elongation of microvillar actin bundles directly through its actin-binding WH2 domain and indirectly through its protein-binding SH3 domain. Additionally, PACSIN2 was found to control microvillar membrane coverage by promoting the targeting of endocytic machinery to the microvillar base. These results show novel roles for BAR domain-containing proteins in the enterocyte brush border and their importance for microvillar growth and maintenance.