Structural characterization of the transmembrane domain from subunit e of yeast F1F0-ATP synthase: A helical GXXXG motif located just under the micelle surfaces

摘要

F1F0-ATP synthase is a large multiprotein complex, including at least 10 subunits in the membrane-bound F-0-sector. One of these F-0 proteins is subunit a (Su e), involved in the stable dimerization of F1F0-ATP synthase, and required for the establishment of normal cristae membrane architecture. As a step toward enabling structure-function studies of the F-0-sector, the Su a transmembrane region was structurally characterized in micelles. Based on a series of NMR and CD (circular dichroism) studies, a structural model of the Su e/micelle complex was constructed, indicating Su a is largely helical, and emerges from the micelle with Arg20 near the phosphate head groups. Su a only adopts this folded conformation in the context of the micelle, and is essentially disordered in DMSO, water or trifluoroethanol/water. Within the micelle the C-terminal Ala10-Arg20 stretch is helical, while the region N-terminal may be transiently helical, based on negative CSI (chemical shift index) values. The Ala10-Arg20 helix contains the G(14)XXXG(18) motif, which has been proposed to play an important role in dimer formation with another protein from the F-0-sector. The Gly on the C-terminal end of this motif (Gly18) is slightly more mobile than the more buried Gly14, based on NMR order parameter measurements (Gly14 S-2 = 0.950; Gly18 S-2 = 0.895). Only one Su a transmembrane peptide is bound per micelle, and micelles are 22-23 angstrom in diameter, composed of 51 +/- 4 dodecylphosphocholine detergent molecules. Although there is no evidence for Su a homodimerization via the transmembrane domain, potentially synergistic roles for N-terminal (membrane) and C-terminal (soluble) domain interactions may still occur. Furthermore, the presence of a buried charged residue (Arg7) suggests there may be interactions with other F-0-sector protein(s) that stabilize this charge, and possibly drive the folding of the N-terminal 9 residues of the transmembrane domain.