1.A.82. The LHFPL Tetraspan Protein (LTSP) Family
Mammalian hair cells are mechanosensors for the perception of sound, acceleration, and fluid motion. Mechanotransduction channels in hair cells are gated by tip links, which connect the stereocilia of a hair cell in the direction of their mechanical sensitivity. The molecular constituents of the mechanotransduction channels of hair cells are numerous. Xiong et al. (2012) showed that mechanotransduction is impaired in mice lacking the tetraspan membrane protein of hair cell stereocilia, TMHS, also known as lipoma HMGIC fusion partner-like 5, LHFPL5). However, TMC1 is likely to be a pore-forming subunit of the transduction channel of cochlear hair cells that is mechanically gated by tension on tip links in the stereocilia bundle (Mahendrasingam and Furness 2019).
TMHS binds to the tip-link component PCDH15 and regulates tip-link assembly, a process that is disrupted by deafness-causing Tmhs mutations. TMHS also regulates transducer channel conductance and is required for fast channel adaptation. TMHS therefore resembles other ion channel regulatory subunits such as the transmembrane alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor regulatory proteins (TARPs) of AMPA receptors that facilitate channel transport and regulate the properties of pore-forming channel subunits. TMHS is an integral component of the hair cell's mechanotransduction machinery that functionally couples PCDH15 to the transduction channel (Xiong et al. 2012). Tmc1 and Tmc2 (TC#s 1.A.17.4.6 and 1.A.17.4.1, respectively) play important roles and are required for normal function of cochlear hair cells, possibly as Ca2+ channels or Ca2+ channel subunits (Kim and Fettiplace 2013). Direct interactions between TMHS, PCDH15 and Tmc1 and Tmc2 have been demonstrated (Maeda et al. 2014; Beurg et al. 2015).
Mechanotransduction channels in hair cells are gated by tip links. Zhao et al. 2014 showed that the transmembrane inner ear protein, TMIE (TC# 9.A.30), forms a ternary complex with the tip-link component PCDH15 and its binding partner TMHS/LHFPL5 (TC# 1.A.82). Alternative splicing of the PCDH15 cytoplasmic domain regulates formation of this ternary complex. Transducer currents are abolished by a homozygous Tmie-null mutation, and subtle Tmie mutations that disrupt interactions between TMIE and tip links affect transduction, suggesting that TMIE is an essential component of the hair cell's mechanotransduction machinery that functionally couples the tip link to the transduction channel. The multisubunit composition of the transduction complex and the regulation of complex assembly by alternative splicing is likely critical for regulating channel properties in different hair cells and along the cochlea's tonotopic axis (Zhao et al. 2014).
Auditory hair cells are restricted to vertebrates (Lumpkin et al. 2010), and the members listed under TC # 1.A.82 include proteins from non-vertebrates including the salmon louse (D3PJ72), the water flea (E9FU04) and the european centipede (T1INH4 ). Furthermore, there is uncertainty about the subunit composition of the vertebrate hair cell mechanotransduction channel (see Wu and Müller 2016 and Fettiplace 2016). For this reason, we have changed the name of this family from the 'Hair Cell Mechanotransduction Channel' (HCMC) (Chou et al. 2017) to the “LHFPL Tetraspan Protein” (LTSP) Family. This name is based on shared domain/structures rather than and uncertain biochemical function (Paul Denny, personal communication).
References:
Mechanotransduction channel complex of cochlear hair cells TMHS/LHFPL5 is encoded by the Lhfpl5 gene. The complex contains several proteins: the tetraspan membrane protein of hair cell stereocilia, (TMHS protein) or Lipoma HMGIC fusion partner-like 5 protein (LHFPL5) (Fettiplace 2016), the Protocadherin-15 protein, PCDH15, and the Tmc1 and Tmc2 proteins (TC# 1.A.17) (Xiong et al. 2012). TMHS and PCDH15 interact directly with Tmc1 and Tmc2, and these interactions are required for mechanotransduction (Maeda et al. 2014; Beurg et al. 2015). A primary function of Tmc1 may be calcium transport (Beurg et al. 2015). Hair cells express two molecularly and functionally distinct mechanotransduction channels with different subcellular distributions. One is activated by sound and is responsible for sensory transduction. This sensory transduction channel is expressed in hair cell stereocilia, and its activity is affected by mutations in the genes encoding the transmembrane proteins TMHS (this family), TMIE (TC family 8.A.116), TMC1 and TMC2 (family 1.A.17.4) (Wu et al. 2016). Thus, these 4 proteins may all be parts of a single channel complex. The other channel is the Piezo2 channel (TC# 1.A.75.1.2). TMHS is 68% identical to the human LHFPL3 protein (Q86UP9), and 62% identical to the human LHFPL4 protein (Q7Z7J7). The structure of protocadherin 15 with the tetraspan, LHFP5, has been determined (Ge et al. 2018). Deafness mutation D572N of TMC1 destabilizes TMC1 expression by disrupting LHFPL5 binding (Yu et al. 2020).
Animals
The mechanotransduction channel complex of Homo sapiens:
Tetraspan TMHS (Q8TAF8)
Protocadherin-15 (Q96QU1)
Lipoma HMGIC fusion partner-like 2 protein of 220 aas and 4 TMSs. The Human ortholog has UniProt acc # Q6ZUX7 with 228 aas and 85% identity with the mouse protein.
Animals
Lipoma HMGIC fusion partner-like 2 protein of Mus musculus (Q8BGA2)
Lipoma HMGIC fusion partner-like 2 protein, LHPL2
Animals
LHPL2 of Lepeophtheirus salmonis (salmon louse)
Hypothetical protein of 299 aas
Animals
HP of Daphnia pulex
Uncharacterized protein of 218 aas and 4 TMSs.
UP of Capitella teleta (Polychaete worm)
The Lipoma HMGIC fusion partner, LHFP of 200 aas and 4 TMSs. Acts as a translocation partner of HMGIC in a lipoma (Petit et al. 1999).
LHFP of Homo sapiens
Tetraspan protein LHFPL4 or GARLH4 of 247 aas and 4 TMSs. It is a synapse-specific tetraspanin essential for inhibitory synapse function because it promotes cell-type specific targeting and clustering of synaptic GABA recpetors (Davenport et al. 2017; Han et al. 2020).
LHFPL4 of Homo sapiens
Uncharacterized protein of 618 aas and possibly 10 - 11 TMSs in a 1 + 2 + 1 + 1 +3 + 2 or 3 TMS arrangement. The N-terminal domain shows sequence similarity to members of TC# 1.A.82 while a central part shows similarity to members of TC# 9.B.422.
UP of Brachionus calyciflorus