Isoform-specific targeting properties of the protocadherin CDHR5 control its apical delivery to promote brush border assembly

Transporting epithelial cells of the gut and kidney interact with their luminal environment through a densely-packed collection of apical microvilli known as the brush border. Proper brush border assembly depends on the intermicrovillar adhesion complex (IMAC), a protocadherin-based adhesion complex found at the distal tips of microvilli that mediates adhesion between neighboring protrusions to promote their organized packing. Loss of the IMAC adhesion molecule Cadherin-related family member 5 (CDHR5) correlates with poor prognosis of colon cancer patients, though the functional properties of this protocadherin have not been thoroughly explored in relevant cell systems. Here, we show that the two dominant CDHR5 splice isoforms expressed in enterocytes interact to form an apparent cis-oligomer that is competent to target to the apical domain to drive microvillar elongation. The two isoforms exhibited distinct sequence-dependent apical targeting properties, with one isoform requiring its cytoplasmic tail. Library screening identified the Ezrin-associated scaffolds EBP50 and E3KARP as cytoplasmic binding partners for CDHR5. Consistent with this, loss of EBP50 disrupted proper brush border assembly with cells exhibiting markedly reduced apical IMAC levels. Together, our results shed light on the apical targeting determinants of CDHR5 and further define the interactome of the IMAC involved in brush border assembly.