Specialized transporting and sensory epithelial cells employ homologous protocadherin-based adhesion complexes to remodel their apical membrane protrusions into organized functional arrays. Within the intestine, the nutrient-transporting enterocytes utilize the intermicrovillar adhesion complex (IMAC) to assemble their apical microvilli into an ordered brush border. The IMAC bears remarkable homology to the Usher complex, whose disruption results in the sensory disorder type 1 Usher syndrome (USH1). However, the entire complement of proteins that comprise both the IMAC and Usher complex are not yet fully elucidated. Using a protein isolation strategy to recover the IMAC, we have identified the small EF-hand protein calmodulin-like protein 4 (CALML4) as an IMAC component. Consistent with this finding, we show that CALML4 exhibits marked enrichment at the distal tips of enterocyte microvilli, the site of IMAC function, and is a direct binding partner of the IMAC component myosin-7b. Moreover, distal tip enrichment of CALML4 is strictly dependent upon its association with myosin-7b, with CALML4 acting as a light chain for this myosin. We further show that genetic disruption of CALML4 within enterocytes results in brush border assembly defects that mirror the loss of other IMAC components and that CALML4 can also associate with the Usher complex component myosin-7a. Our study further defines the molecular composition and protein-protein interaction network of the IMAC and Usher complex and may also shed light on the etiology of the sensory disorder USH1H.

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.

MyTH4-FERM (MF) myosins evolved to play a role in the creation and function of a variety of actin-based membrane protrusions that extend from cells. Here, we performed an analysis of the MF myosins, Myo7A, Myo7B, and Myo10, to gain insight into how they select for their preferred actin networks. Using enterocytes that create spatially separated actin tracks in the form of apical microvilli and basal filopodia, we show that actin track selection is principally guided by the mode of oligomerization of the myosin along with the identity of the motor domain, with little influence from the specific composition of the lever arm. Chimeric variants of Myo7A and Myo7B fused to a leucine zipper parallel dimerization sequence in place of their native tails both selected apical microvilli as their tracks, while a truncated Myo10 used its native antiparallel coiled-coil to traffic to the tips of filopodia. Swapping lever arms between the Class 7 and 10 myosins did not change actin track preference. Surprisingly, fusing the motor-neck region of Myo10 to a leucine zipper or oligomerization sequences derived from the Myo7A and Myo7B cargo proteins USH1G and ANKS4B, respectively, re-encoded the actin track usage of Myo10 to apical microvilli with significant efficiency.

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.