Nonlinear compliance of NOMPC gating spring and its implication in mechanotransduction.

NOMPC is a type of mechanosensitive ion channel (MSCs) that are tethered to cytoskeleton through gating springs and gated by forces applied to the cell, thereby mediating processes including hearing, touch sensation, and proprioception. The mechanical properties of the gating spring play crucial roles in force transmission and channel opening and adaptation, but have been poorly characterized so far. Here, we measured the stiffness and unfolding of ankyrin-repeat domains (ARDs), the gating spring in the tetrameric NOMPC complex, by pulling single or multiple ARDs using high-resolution optical tweezers. We found that each ARD has a low stiffness of ∼0.7 pN/nm and starts to unfold stepwise at ∼7 pN. Four ARDs show cooperativity in their mechanical properties through interactions at their lower contact sites. This find is further supported by cryo-EM structures of NOMPC containing full-length and truncated ARDs. Overall, our work demonstrates a compliant and unfolding-refolding gating spring essential for the function of NOMPC and likely other MSCs.