The second PI(3,5)P 2 binding site in the S0 helix of KCNQ1 stabilizes PIP 2 -at the primary PI1 site with potential consequences on intermediate-to-open state transition.

The Phosphatidylinositol 3-phosphate 5-kinase Type III PIKfyve is the main source for selectively generated phosphatidylinositol 3,5-bisphosphate (PI(3,5)P ₂ ), a known regulator of membrane protein trafficking. PI(3,5)P ₂ facilitates the cardiac KCNQ1/KCNE1 channel plasma membrane abundance and therewith increases the macroscopic current amplitude. Functional-physical interaction of PI(3,5)P ₂ with membrane proteins and its structural impact is not sufficiently understood. This study aimed to identify molecular interaction sites and stimulatory mechanisms of the KCNQ1/KCNE1 channel via the PIKfyve-PI(3,5)P ₂ axis. Mutational scanning at the intracellular membrane leaflet and nuclear magnetic resonance (NMR) spectroscopy identified two PI(3,5)P ₂ binding sites, the known PIP ₂ site PS1 and the newly identified N-terminal α-helix S0 as relevant for functional PIKfyve effects. Cd ²⁺ coordination to engineered cysteines and molecular modeling suggest that repositioning of S0 stabilizes the channel s open state, an effect strictly dependent on parallel binding of PI(3,5)P ₂ to both sites.
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