Structure of Myo7b/USH1C complex suggests a general PDZ domain binding mode by MyTH4-FERM myosins

Abstract

Unconventional myosin 7a (Myo7a), myosin 7b (Myo7b), and myosin 15a (Myo15a) all contain MyTH4-FERM domains (myosin tail homology 4-band 4.1, ezrin, radixin, moesin; MF) in their cargo binding tails and are essential for the growth and function of microvilli and stereocilia. Numerous mutations have been identified in the MyTH4-FERM tandems of these myosins in patients suffering visual and hearing impairment. Although a number of MF domain binding partners have been identified, the molecular basis of interactions with the C-terminal MF domain (CMF) of these myosins remains poorly understood. Here we report the high-resolution crystal structure of Myo7b CMF in complex with the extended PDZ3 domain of USH1C (a.k.a., Harmonin), revealing a previously uncharacterized interaction mode both for MyTH4-FERM tandems and for PDZ domains. We predicted, based on the structure of the Myo7b CMF/USH1C PDZ3 complex, and verified that Myo7a CMF also binds to USH1C PDZ3 using a similar mode. The structure of the Myo7b CMF/USH1C PDZ complex provides mechanistic explanations for >20 deafness-causing mutations in Myo7a CMF. Taken together, these findings suggest that binding to PDZ domains, such as those from USH1C, PDZD7, and Whirlin, is a common property of CMFs of Myo7a, Myo7b, and Myo15a.M icrovilli and stereocilia are both actin bundle-based, fingerlike protrusions found on apical surfaces of many epithelial cells (1). Microvilli line the apical surface of epithelial cells forming a densely packed structure known as the brush border in tube-like tissues, such as intestines, kidney, and lung (2-5). The best known function of microvilli is to massively increase membrane surface area of these tissues to promote solute exchange, although these protrusions may also allow cells to communicate with their extracellular surroundings (6). Stereocilia, on the other hand, are composed of several rows of protrusions with graded height forming a staircase-like structure on the apical surface of inner ear hair cells, which collectively function to sense sound waves (7,8). Despite clear morphological and functional differences, microvilli and stereocilia share certain features at the molecular level. For example, the packing and organization of microvilli and stereocilia both rely on homologous cadherin-based tip-link complexes that target to the distal ends of these protrusions (9, 10). Additionally, a set of homologous unconventional myosins-including Myo1, Myo3, Myo6, and Myo7-are shared by and critical for the development, maintenance, and functions of microvilli and stereocilia (1,11,12).This study focuses on Myo7b, an unconventional myosin enriched in the microvilli of transporting epithelial cells (11, 13). Myo7b is characterized by a pair of MyTH4-FERM tandems in its tail cargo-binding domain. In addition to Myo7b, Myo7a, Myo10, and Myo15a also contain one or two MyTH4-FERM tandems in their tail domains (14). A common property of MyTH4-FERM myosins is their involvement in the formation or elongation/stabilization of actin-bundle support...

Regulation of actin-based apical structures on epithelial cells

“…1A) (Pickles et al, 1984). Collectively, this work has shown that the MyTH4-FERM tail domain of the motor protein Myo7a is essential to form a stable ternary complex with the PDZ-domain-containing scaffolding proteins USH1C (harmonin) and scaffold protein containing ankyrin repeats and SAM domain (SANS, also called USH1G) to bind the cytoplasmic tail of the cadherin CDH23 (Caberlotto et al, 2011;Grati and Kachar, 2011;Li et al, 2017). In addition, the extracellular region of the cadherin dimer interacts with the extracellular region of the protocadherin PCDH15 dimer on a neighboring stereocilium (Siemens et al, 2004).…”

The small EF-hand protein CALML4 functions as a critical myosin light chain within the intermicrovillar adhesion complex

“…Of note, the causative genes for three USH1 subtypes (USH1E, USH1K and USH1H) still have not been identified, suggesting that more Usher complex components may remain to be discovered (17)(18)(19). A similar situation is seen with the IMAC; structure-function studies of this complex clearly point towards the idea that the full interactome is not yet known (5,20,21).…”

Myosin VII, USH1C, and ANKS4B or USH1G Together Form Condensed Molecular Assembly via Liquid-Liquid Phase Separation

“…Immunostaining studies showed that Myosin VIIA (MYO7A), USH1G (also known as Sans), and USH1C (also known as Harmonin or autoimmune enteropathy-related 75-kDa antigen ) are located at the UTLD, suggesting that they are involved in the electron-dense plaque formation, although the mechanism underlying the dense assembly formation is not understood (Grati and Kachar, 2011;Grillet et al, 2009). MYO7A, USH1G, and USH1C form a very stable tripartite complex via several pairs of specific interactions (Adato et al, 2005;Li et al, 2017;Pan and Zhang, 2012;Wu et al, 2011;Yan et al, 2010;Yu et al, 2017). The cytoplasmic tail (CT) of CDH23 interacts with USH1C (Pan et al, 2009;Siemens et al, 2002), thus linking the tail of the upper tip-link to UTLD ( Figure 1A).…”

“…The cadherin-related family member 2 (CDHR2, also known as protocadherin 24) and cadherin-related family member 5 (CDHR5, also known as mucin-like protocadherin) interact in trans to form the filamentous tip-link (Crawley et al, 2014b). The CT of CDHR2 is coupled to the actin filaments via a tripartite complex composed of Myosin VIIB (MYO7B), USH1C, and ANKS4B (Crawley et al, 2014b(Crawley et al, , 2016Li et al, 2016aLi et al, , 2017Weck et al, 2016;Yu et al, 2017).…”