Analysis of the mechanosensor channel functionality of TACAN

Abstract

Mechanosensitive ion channels mediate transmembrane ion currents activated by mechanical forces. A mechanosensitive ion channel called TACAN was recently reported. We began to study TACAN with the intent to understand how it senses mechanical forces and functions as an ion channel. Using cellular patch-recording methods we failed to identify mechanosensitive ion channel activity. Using membrane reconstitution methods we found that TACAN, at high protein concentrations, produces heterogeneous conduction levels that are not mechanosensitive and are most consistent with disruptions of the lipid bilayer. We determined the structure of TACAN using single particle cryo-EM and observe that it forms a symmetrical dimeric transmembrane protein. Each protomer contains an intracellular-facing cleft with a coenzyme-A cofactor, confirmed by mass spectrometry. The TACAN protomers are related in 3-dimensional structure to a fatty acid elongase, ELOVL. Whilst its physiological function remains unclear, we anticipate that TACAN is not a mechanosensitive ion channel.

Cryo-EM structure of the human TACAN channel in a closed state

“…Recently published literatures argued that the TACAN may be a coenzyme A-binding protein rather than a mechanosensitive channel [21][22][23]. They showed that a coenzyme A molecule is bound in the vestibule of the membrane domain of hTACAN and also the expression of hTACAN was not sufficient in mediating mechanosensitive currents in cells.…”

Pain or gain?

“…By conclusively showing that TMEM120A specifically interacts with CoASH, the work of Rong et al suggests that this membrane protein might in fact work as a membrane-embedded enzyme. It is also notable that Rong et al, along with several other groups ( Ke et al, 2021 ; Niu et al, 2021 ; Parpaite et al, 2021 ; Rong et al, 2021 ; Xue et al, 2021 ), could not reproduce the mechanosensitive currents reported previously. Moreover, there are several lines of evidence to suggest that TMEM120A has a greater role in lipid metabolism.…”

“…Several recent reports have shown that C. elegans and mammalian cells need it to accumulate fat, and that disrupting the gene for TMEM120A leads to metabolic defects in mice fed a high-fat diet ( Czapiewski et al, 2021 ; Li et al, 2021 ). In addition, its structure also resembles that of ELOVL7, a membrane-embedded enzyme that helps to elongate fatty acids ( Nie et al, 2020 ; Niu et al, 2021 ; Xue et al, 2021 ). However, TMEM120A does not catalyze the same reactions as ELOVL7, and the possible substrates and end products of TMEM120A remain unknown ( Niu et al, 2021 ).…”

“…In addition, its structure also resembles that of ELOVL7, a membrane-embedded enzyme that helps to elongate fatty acids ( Nie et al, 2020 ; Niu et al, 2021 ; Xue et al, 2021 ). However, TMEM120A does not catalyze the same reactions as ELOVL7, and the possible substrates and end products of TMEM120A remain unknown ( Niu et al, 2021 ).…”

Regulation of the PKD2 Channel Function by TACAN

“…TMEM120A (transmembrane protein 120A, also called TACAN) was initially reported as a nuclear envelope transmembrane protein critical for adipocyte differentiation (Batrakou et al, 2015). It was recently shown to form a mechano-sensitive ion channel sensing the pain (Beaulieu-Laroche et al, 2020), but this was challenged by subsequent structural and functional studies (Del Rosario et al, 2021; Ke et al, 2021; Niu et al, 2021; Parpaite et al, 2021; Rong et al, 2021; Xue et al, 2021). Global knockout of TACAN led to embryo death indicating its importance for embryonic development (Beaulieu-Laroche et al, 2020).…”