Membrane integration of Na,K-ATPase α-subunits and β-subunit assembly

Abstract

The control of membrane insertion of polytopic proteins is still poorly understood. We carried out in vivo translation/insertion experiments in Xenopus oocytes with combined wild type or mutant membrane segments of the α-subunit of the heterodimeric Na,K-ATPase linked to a glycosylation reporter sequence. We confirm that the four N-terminal hydrophobic segments of the α-subunit behave as alternating signal anchor/stop transfer motifs necessary for two lipid-inserted membrane pairs. For the six C-terminal membrane segments, however, proper packing depends on specific sequence information and association with the β-subunit. M5 is a very inefficient signal anchor sequence due to the presence of prolines and polar amino acids. Its correct membrane insertion is probably mediated by posttranslational hairpin formation with M6, which is favored by a proline pair in the connecting loop. M7 has partial signal anchor function, which may be mediated by the presence of glycine and glutamine residues. The formation of a transmembrane M7/M8 pair requires the association of the β-subunit, which induces a conformational change in the connecting extracytoplasmic loop that favors M7/M8 packing. The formation of the M9/M10 pair appears to be predominantly mediated by the efficient stop transfer function of M10. Mutation that provide signal anchor function to M5, M7, and M9 abolish or impede the transport activity of the enzyme. These data illustrate the importance of specific amino acids near or within hydrophobic regions as well as of subunit oligomerization for correct topographical alignment that is necessary for proper folding and/or activity of oligomeric membrane proteins.