3.A.3.8.16
ATP9A lipid flippase of 1047 aas and 10 TMSs.  It is present in the liver canicular membrane (Chaubey et al. 2016). Among mammalian P4-ATPase flippases, only ATP9A and ATP9B do not require the auxiliary subunit CDC50 protein. While its yeast homologue, Neo1, is essential for cell survival, little is known about mammalian ATP9A. Cryo-EM structures of human monomeric ATP9A were otained at a resolution reaching to 2.2Å, in the outward-facing E2P state (Abe et al. 2025). Two distinguishable conformations were obtained from a single sample, one with its outward gate open, and the other in its closed form. Unlike canonical gating observed for most P-type ATPases, which is driven by the movement of transmembrane (TM) helices 1 and 2 linked to the A domain, outward gating in ATP9A is achieved by the movement of TMSs 6-10, likely initiated by the unwinding of TMS 6. As a result, the volume of the phospholipid binding cavity in the open state surpasses that of other flippases, which could allow binding of phospholipids with larger hydrophilic head groups than that of phosphatidylserine. ATP9A shows an ATPase activity that is increased by the addition of certain phospholipids. This provide mechanistic rationales for ATP9A gating, achieved by the rearrangement of the second half of the TM helices. Since TMS4 - TMS10 is anchored by the CDC50 protein subunit in other flippases, the outward gating mechanism is unique to P4B-type flippases which function as a monomer (Abe et al. 2025).