Fig. 1. The USH1 proteins and their interactions in vitro. (A)
Predicted structures of the different USH1 protein isoforms. Myosin VIIa
consists of a motor head, a neck region with five isoleucine-glutamine (IQ)
motifs, and a large tail comprising an 
-helix domain and two repeats,
each composed of a myosin tail homology 4 (MyTH4) domain and a 4.1 ezrin
radixin moesin (FERM) domain, separated by a Src homology 3 (SH3) domain
(Chen et al., 1996;
Weil et al., 1995). There are
three classes of harmonin isoforms (a, b and c)
(Verpy et al., 2000).
Harmonin-a and harmonin-c have three and two PDZ domains, respectively, and
harmonin-a also has a coiled-coil (CC) domain. Harmonin-b has the same domains
as harmonin-a, plus a second CC domain and a proline, serine and threonine
(PST)-rich sequence. Cadherin 23 and protocadherin 15 isoforms are also
grouped into three classes (Ahmed et al.,
2006; Lagziel et al.,
2005). Cadherin 23a, cadherin 23b, protocadherin 15a and
protocadherin 15b are transmembrane isoforms, with 27, 7, 11 and 1
extracellular cadherin (EC) repeat, respectively. Cadherin 23c isoforms are
cytoplasmic, whereas protocadherin 15si isoforms are secreted. Multiple splice
variants have been identified for myosin VIIa, protocadherin 15a, and each
isoform class of harmonin and cadherin 23 (alternative exons are indicated)
(Ahmed et al., 2006;
Ahmed et al., 2003;
Chen et al., 1996;
Lagziel et al., 2005;
Michel et al., 2005;
Reiners et al., 2003;
Verpy et al., 2000;
Weil et al., 1995). Finally,
sans has three ankyrin (ANK)-like repeats and a sterile alpha motif (SAM)
domain (Kikkawa et al., 2003;
Weil et al., 2003). Sans does
not have any known splice variants. The respective locations of the mutations
of the five USH1 mouse models used in this study are indicated by arrows for
point mutations, and by inhibition signs for the deletion (del) of the
transcription start site. The resulting stop codons are indicated by
asterisks. The immunogenic regions for the antibodies used in this study are
indicated by brown boxes. (B) Apical views of the auditory epithelium
of a P5 wild-type mouse by scanning electron microscopy (SEM). Sensory inner
hair cells (IHCs) and outer hair cells (OHCs) are organized into a single
medial-side row and three lateral-side rows, respectively (left panel). A hair
bundle that consists of stereocilia and a single transient kinocilium is
present on top of every hair cell (right). Scale bar: 1 µm. (C)
Schematic representation of known in vitro interactions between the USH1
proteins. The domains involved in each interaction are drawn in close
apposition. Harmonin can bind to any of the other USH1 proteins. Cadherin 23a
and protocadherin 15a/b cytoplasmic regions interact with harmonin PDZ1 and/or
PDZ2 domains (Adato et al.,
2005; Boeda et al.,
2002; Reiners et al.,
2005; Siemens et al.,
2002). The presence of a consensus PDZ-binding motif at the
C-terminus of cadherin 23a and protocadherin 15a/b isoforms is indicated by a
star. Through its cytoplasmic region, protocadherin 15a/b can also bind to the
myosin VIIa SH3 domain (Senften et al.,
2006). The harmonin PDZ1 domain can interact with the second
MyTH4-FERM repeat of the myosin VIIa tail and the SAM domain of sans
(Boeda et al., 2002;
Weil et al., 2003). Finally,
the central region of sans can bind to the first MyTH4-FERM repeat of myosin
VIIa (Adato et al., 2005).
Harmonin and sans can also form homodimers (not shown)
(Adato et al., 2005).