ATP8A2 is a P4-type ATPase, a ”flippase” that translocates aminophospholipids such as phosphatidylserine (PS) and phosphatidylethanolamine (PE) from the exoplasmic to the cytoplasmic side of the plasma membrane. This flippase has similar mechanistic characteristics as the ion-translocating P2-type ATPases, though it catalyzes the transport of a much larger substrate, an enigma referred to as the “giant substrate problem”. Recently, based on mutational analysis and molecular dynamics we have identified a hydrophobic gate in a groove surrounded by M1, M2, M4 and M6 (1). A plausible water filled pocket moves in the groove, when the enzyme shifts between the two states E2P and E2, and the aminophospholipid headgroup may be located in the pocket during the transport, with the lipid tail being dragged along, jutting out into the lipid bilayer, the so-called “credit card model”. Hydrophobic residues, especially I364 in M4, surrounding this pocket are involved in releasing the substrate in the transformation of E2 to E1 state. Previously, K873, located in M5, was found involved directly or indirectly in defining the substrate affinity in ATP8A2 (2). In the present work we identify a stability cluster harboring key residues in the center of the enzyme important for linking M5 through K873 to M4 and M6. This stability cluster supposedly allows M4 to act as a pumping rod during enzyme reaction cycle. We find that mutation of residues in this stability cluster affects the substrate affinity, as previously shown for K873. Hence, the stability cluster appears important in linking the hydrophobic gate to K873 in M5.
Referencer

1. Vestergaard, A. L., Coleman, J. A., Lemmin, T., Mikkelsen, S. A., Molday, L. L., Vilsen, B., Molday, R. S., Dal, P. M., and Andersen, J. P. (2014) Proc. Natl. Acad. Sci. U. S. A 111, E1334-E1343
2. Coleman, J. A., Vestergaard, A. L., Molday, R. S., Vilsen, B., and Andersen, J. P. (2012) Proc. Natl. Acad. Sci. U. S. A 109, 1449-1454