Cadherin-23, myosin VIIa and harmonin, encoded by Usher syndrome type I genes, form a ternary complex and interact with membrane phospholipids

Abstract

Cadherin-23 is a component of early transient lateral links of the auditory sensory cells' hair bundle, the mechanoreceptive structure to sound. This protein also makes up the upper part of the tip links that control gating of the mechanoelectrical transduction channels. We addressed the issue of the molecular complex that anchors these links to the hair bundle F-actin core. By using surface plasmon resonance assays, we show that the cytoplasmic regions of the two cadherin-23 isoforms that do or do not contain the exon68-encoded peptide directly interact with harmonin, a submembrane PDZ (post-synaptic density, disc large, zonula occludens) domain-containing protein, with unusually high affinity. This interaction involves the harmonin Nter-PDZ1 supramodule, but not the C-terminal PDZ-binding motif of cadherin-23. We establish that cadherin-23 directly binds to the tail of myosin VIIa. Moreover, cadherin-23, harmonin and myosin VIIa can form a ternary complex, which suggests that myosin VIIa applies tension forces on hair bundle links. We also show that the cadherin-23 cytoplasmic region, harmonin and myosin VIIa interact with phospholipids on synthetic liposomes. Harmonin and the cytoplasmic region of cadherin-23, both independently and as a binary complex, can bind specifically to phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2), which may account for the role of this phospholipid in the adaptation of mechanoelectrical transduction in the hair bundle. The distributions of cadherin-23, harmonin, myosin VIIa and PI(4,5)P2 in the growing and mature auditory hair bundles as well as the abnormal locations of harmonin and myosin VIIa in cadherin-23 null mutant mice strongly support the functional relevance of these interactions.

Usher type 1G protein sans is a critical component of the tip-link complex, a structure controlling actin polymerization in stereocilia

“…The direct in vitro interactions between the five USH1 proteins (2,(12)(13)(14)(15)(16) and the colocalization of cadherin-23, protocadherin-15, myosin VIIa, and harmonin in the growing hair bundle, along with the common early morphological defect in cochlear hair bundles of mouse mutants defective for any USH1 gene (17), suggest that USH1 proteins cooperate in hair bundle development. It has been proposed that harmonin-b anchors transient fibrous lateral links to the stereocilia actin filaments and that myosin VIIa creates tension on these links (2,16), but the role played by sans is still unknown. Sans directly interacts in vitro with both the myosin VIIa tail (13) and harmonin (13,18), but has so far not been detected within the hair bundle (13), thus questioning its participation in the USH1 molecular network that underlies cohesion of the growing hair bundle.…”

Cargo recognition mechanism of myosin X revealed by the structure of its tail MyTH4-FERM tandem in complex with the DCC P3 domain

“…These three myosins are so named because the tail of each motor contains one or a pair of MyTH4-FERM domains. The MyTH4-FERM domains in the tail regions of myosin VII, X, and XV are believed to function as cargo binding domains of these myosins (1)(2)(3)(4)(5)(6)(7), although the molecular basis of the motor-cargo interactions are not known.…”

Listen to your gut: Using adhesion to shape the surface of functionally diverse epithelia

“…26 CDH23 forms the upper portion of the tip-link, where its cytoplasmic domain interacts directly with USH1C, which in turn interacts with the ANKS4B-like protein, SANS, and the class VII myosin, MYO7A. [27][28][29][30][31][32][33] PCDH15 forms the lower portion of the tip-link and is believed to interact with a mechanotransduction channel that allows hair cells to respond to physical forces. 34,35 Consistent with studies on the IMAC, formation of the tip-link complex also appears to be critical for hair bundle organization.…”