Scaffold protein harmonin (USH1C) provides molecular links between Usher syndrome type 1 and type 2

Abstract

Usher syndrome (USH) is the most frequent cause of combined deaf-blindness in man. USH is clinically and genetically heterogeneous with at least 11 chromosomal loci assigned to the three USH types (USH1A-G, USH2A-C, USH3A). Although the different USH types exhibit almost the same phenotype in human, the identified USH genes encode for proteins which belong to very different protein classes and families. We and others recently reported that the scaffold protein harmonin (USH1C-gene product) integrates all identified USH1 molecules in a USH1-protein network. Here, we investigated the relationship between the USH2 molecules and this USH1-protein network. We show a molecular interaction between the scaffold protein harmonin (USH1C) and the USH2A protein, VLGR1 (USH2C) and the candidate for USH2B, NBC3. We pinpoint these interactions to interactions between the PDZ1 domain of harmonin and the PDZ-binding motifs at the C-termini of the USH2 proteins and NBC3. We demonstrate that USH2A, VLGR1 and NBC3 are co-expressed with the USH1-protein harmonin in the synaptic terminals of both retinal photoreceptors and inner ear hair cells. In hair cells, these USH proteins are also localized in the signal uptaking stereocilia. Our data indicate that the USH2 proteins and NBC3 are further partners in the supramolecular USH-protein network in the retina and inner ear which shed new light on the function of USH2 proteins and the entire USH-protein network. These findings provide first evidence for a molecular linkage between the pathophysiology in USH1 and USH2. The organization of USH molecules in a mutual 'interactome' related to the disease can explain the common phenotype in USH.

Usherin is required for maintenance of retinal photoreceptors and normal development of cochlear hair cells

“…Others recently have proposed that usherin localizes and functions at the photoreceptor and hair-cell synapses, in conjunction with other Usher proteins (4,17,18). Our present study finds no evidence for the presence of usherin at the photoreceptor synapse or a defective synaptic transmission from photoreceptors to second-order retinal neurons.…”

“…The spatially restricted distribution of usherin at this patch of plasma membrane marks it as a specialized microdomain and supports the view that a functional equivalent periciliary ridge complex does exist in mammals. Usherin is likely to be tethered via its C-terminal PDZ-binding motif to this membrane microdomain through protein-protein interaction with PDZ domain proteins (16)(17)(18)) (see Fig. 3c) (unpublished observations).…”

“…There have been conflicting reports about usherin tissue distribution and subcellular localization (4,13,(16)(17)(18). The putative 600-kDa full-length protein never was confirmed from native tissues, and the function of usherin was poorly understood.…”

The structure of the harmonin/sans complex reveals an unexpected interaction mode of the two Usher syndrome proteins

“…The stable α-helical N-domain of harmonin, together with its second PDZ domain, bind to two separate fragments of the cadherin23 cytoplasmic domain, providing a structural explanation for the stable harmonin/cadherin23 complex found in the tip links of hair cells (11,16,18). In this study, we first set out to characterize the structure of the harmonin PDZ1, as the domain has been shown to bind canonical carboxyl peptide ligands (e.g., the C-terminal tails of Sans, Usherin, and VLGR1) as well as the SAM domain of Sans and the tail domain of Myosin VIIa (7,8,19). We discovered that the cadherin23-binding N-domain, PDZ1, and a 25-residue extension C-terminus to PDZ1 (residues , and referred to as NPDZ1) forms an integral structural and functional supramodule (Figs.…”

Vezatin, a ubiquitous protein of adherens cell–cell junctions, is exclusively expressed in germ cells in mouse testis

“…How myosin VIIa, exclusively expressed in Sertoli cells, is targeted to the apical ectoplasmic specialization in the absence of one of its transmembrane partner, vezatin, remains to be elucidated. Recent studies indicate that myosin VIIa is integrated together with other actin-binding proteins into complex protein networks (Boeda et al 2002, Adato et al 2005, Reiners et al 2005, Senften et al 2006. In the testis, members of these complexes may target myosin VIIa to the spermatid membrane.…”