Structural impact of proline mutations in the loop region of an ancestral membrane protein

The sodium ion-translocating F0 F1 ATP synthase from the bacterium Ilyobacter tartaricus contains a highly stable rotor ring composed of 11 c subunits. The synthase subunit c-in effect an 89-residue peptide that folds into a helical hairpin consisting of two membrane-spanning helices and a cytoplasmic loop-was probed for the structural impact of a series of substitutions with the β-turn-inducing proline-glycine couplet scanning the hairpin loop (residues 44-51) of the I. tartaricus sequence. We found that a Pro residue in other than the wild type position 47 alters the gross secondary structure of subunit c from α-helical to β-sheet-like, as well as changing its oligomeric ring structure, and its stability toward heat and trichloroacetic acid treatment. Such a Pro-mediated structural switch in one of the first membrane proteins in life hints to a potential evolutionary connection between α-helical and β-sheet membrane proteins.