Structural and functional basis of mechanosensitive TMEM63 channelopathies.

TMEM63A, -B, and -C constitute a mammalian family of mechanosensitive ion channels that are mutated in neurodevelopmental disorders. The molecular mechanisms underlying TMEM63 activation by force and the impact of disease-associated mutations have not been clarified. Here, we elucidate the structural and functional bases of a prevalent TMEM63B mutation p.V44M. We first found that TMEM63B p.V44M and the homologous TMEM63A p.V53M are gain-of-function mutations that do not enhance channel activity but instead evoke constitutive lipid scramblase activity. We then solved TMEM63A p.V53M mutant structures in both closed and lipid-open states, which revealed major rearrangements of pore-lining helices, creating a lateral cleft across the membrane. Simulation studies revealed lipid scrambling through this cleft. The structural rearrangements were triggered by disruption of a surface-proximal hydrophobic latch, a putative force-sensing module that includes a cluster of disease mutation sites. Our findings provide mechanistic insight into TMEM63 channelopathies and suggest a possible force-sensing mechanism.