comparing the structures of the y and a rotor stalk obtained under different conditions or

from different sources, in combination with an overwhelming amount of biochemical and

immunological evidence, it is clear that the domains of the two subunits undergo maj or

shifts in position, which reflects its fundamental role in the synthesis of ATP (20, 44, 48,

71-80).

The y subunit

 In E. coli, the y subunit is the third largest subunit of F1Fo ATP synthase, encoded

by the uncG gene as a 286 residue polypeptide with a deduced molecular weight of

3 1,563 Da. It plays an essential role in coupling proton transport to the synthesis of ATP.

The first visualization of a portion of the y subunit was a bovine Fl partial structure

solved in combination along with the u3 3 hexamer revealing three a-helices (20). The

Y209-272 (TOSidues 223-286 in the E. coli sequence) carboxyl terminus formed a long (90 A+)

a-helix extending from the stalk structure seen by EM to about 15 A+ from the top of the

hexamer. The bottom half of this helix formed a left-handed anti-parallel coiled coil with

a shorter a-helix composed of the amino-terminal residues Yl-45 (20). The two helices

protruded about 30 A+ from the bottom of F1. An approximately 200 kink in the latter

helix was produced by ypro40 and a similar but less pronounced kink was induced by Yleu217

in the former (48). A third, much smaller a-helix, composed of y73-90 (TOSidues 83-99 in

the E. coli sequence) was inclined at about 45 degrees from the larger helices and located

directly under the Fl hexamer.

 More recently, the complete structure of the bovine rotor stalk has been solved to

2.4 A+ (48). The overall length of the stalk, from the carboxyl terminus of the y subunit to

the very bottom where it contacts the ring of c subunits, was 1 14 A+. The portion that