including p21'", adenylate kinase, RecA, elongation factor Tu, and transducin-a (20, 38-

42).

 Tertiary structure. The first high-resolution structure of bovine Fl resolved at the

atomic level (2.8 A+) was solved by Walker's group a decade ago (20). It was found to be

a flattened sphere approximately 80 A+ high and 100 A+ wide with the three a and three P

subunits arranged as a hexamer of alternating subunits around a centrally located 90 A~

long a-helix formed by the y subunit. A dimple 15 A+ deep is located at the top of F1.

The amino-terminal regions of the a and P subunits were once thought to be in close

proximity to the membrane due to labeling experiments (43). Contrary to this early data,

the crystallographic data placed the amino-terminal regions on the top of the u3 3

hexamer over 100 A+ away from the lipid bilayer.

 The folds of the a and P subunits were found to be nearly identical. They each

consisted of a six-stranded P-barrel at the amino terminus (al9-95, P9-82), ai central a-p

domain containing the nucleotide-binding site (u96-379, P83-363) and a bundle of seven and

six helices at the carboxyl termini of the a and P subunits, respectively (u380-510, P364-474)

(20). The nucleotide binding domain consisted of a nine stranded P-sheet with nine

associated a-helices, of which the a-carbons of the seven P-strands and the seven

associated helices can be superimposed onto the RecA protein ATP binding site with an

rms separation of 1.9 A+ (20).

 The three catalytic sites were located at the interfaces of the u3 3 hexamer. In the

original crystal structure, now commonly referred to as the reference structure, two of the

three sites were occupied by nucleotide, containing MgADP ("(PDP Site") and MgAMP-

PN~P ("(PTP Site"). The third site was empty and designated "(PE." The PDP and PTP