Probing the Architecture, Dynamics, and Inhibition of the PI4KIIIα/TTC7/FAM126 Complex.

Phosphatidylinositol 4-kinase IIIα (PI4KIIIα) is the lipid kinase primarily responsible for generating the lipid phosphatidylinositol 4-phosphate (PI4P) at the plasma membrane, which acts as the substrate for generation of the signaling lipids PIP2 and PIP3. PI4KIIIα forms a large heterotrimeric complex with two regulatory partners, TTC7 and FAM126. We describe using an integrated electron microscopy and hydrogen-deuterium exchange mass spectrometry (HDX-MS) approach to probe the architecture and dynamics of the complex of PI4KIIIα/TTC7/FAM126. HDX-MS reveals that the majority of the PI4KIIIα sequence was protected from exchange in short deuterium pulse experiments, suggesting presence of secondary structure, even in putative unstructured regions. Negative stain electron microscopy reveals the shape and architecture of the full-length complex, revealing an overall dimer of PI4KIIIα/TTC7/FAM126 trimers. HDX-MS reveals conformational changes in the TTC7/FAM126 complex upon binding PI4KIIIα, including both at the direct TTC7-PI4KIIIα interface and at the putative membrane binding surface. Finally, HDX-MS experiments of PI4KIIIα bound to the highly potent and selective inhibitor GSK-A1 compared to that bound to the non-specific inhibitor PIK93 revealed substantial conformational changes throughout an extended region of the kinase domain. Many of these changes were distant from the putative inhibitor binding site, showing a large degree of allosteric conformational changes that occur upon inhibitor binding. Overall, our results reveal novel insight into the regulation of PI4KIIIα by its regulatory proteins TTC7/FAM126, as well as additional dynamic information on how selective inhibition of PI4KIIIα is achieved.