A cryptic sequence targets the adhesion complex scaffold ANKS4B to apical microvilli to promote enterocyte brush border assembly.

Nutrient-transporting enterocytes interact with their luminal environment using a densely-packed collection of apical microvilli known as the brush border.  Assembly of the brush border is controlled by the intermicrovillar adhesion complex (IMAC), a protocadherin-based complex found at the tips of brush border microvilli that mediates adhesion between neighboring protrusions.  ANKS4B is known to be an essential scaffold within the IMAC, though its functional properties have not been thoroughly characterized.  We report here that ANKS4B is directed to the brush border using a non-canonical apical targeting sequence that maps to a previously unannotated region of the scaffold.  When expressed on its own, this sequence targeted to microvilli in the absence of any direct interaction with the other IMAC components.  Sequence analysis revealed a coiled-coil motif and a putative membrane-binding basic-hydrophobic repeat sequence within this targeting region, both of which were required for the scaffold to target and mediate brush border assembly.  Size exclusion chromatography of the isolated targeting sequence coupled with in vitro brush border binding assays suggests that it functions as an oligomer.  We further show that the corresponding sequence found in the closest homolog of ANKS4B, the scaffold USH1G that operates in sensory epithelia as part of the Usher complex, lacks the inherent ability to target to microvilli. This study further defines the underlying mechanism of how ANKS4B targets to the apical domain of enterocytes to drive brush border assembly, and identifies a point of functional divergence between the ankyrin repeat-based scaffolds found in the IMAC and Usher complex.