A dimer of two b subunits is the maj or component of the peripheral stalk of F1Fo

ATP synthase and is necessary for normal assembly and function. The b dimer spans the

cytoplasmic membrane and reaches towards the topside of F1 where it meets the 6 subunit

(Figure 4-1) (100, 141-143). A substantial amount of evidence suggests the b dimer to be

at least 80% a-helical, with 14% P-turn, and exist in a parallel, elongated conformation,

spanning about 45 A+ from the top of the membrane to the bottom of the u3 3 hexamer or

about 100 A+ to the top of the hexamer (12, 13, 166, 196, 292). Four domains comprise

the b subunits: the amino-terminal membrane spanning, tether, dimerization and the

carboxyl terminal 6-binding domains (Figure 4-5) (13). Despite the previous model of

the b subunit dimer, describing it as a rigid structural feature of F1Fo ATP synthase,

accumulating evidence suggests a more flexible stalk model (193-195, 202).

 It has been shown by Dr. Paul Sorgen that F1Fo ATP synthase retains sufficient

levels of activity upon relatively large deletions or insertions in the b subunit. An eleven

amino acid deletion and a fourteen amino acid insertion in the region of the b subunit

spanning the tether domain and the beginning of the dimerization domain was

accommodated by the enzyme (Figure 4-5). Assuming a-helical structure, this 21 A+

insertion and 16 A+ deletion corresponds to well over a third of the length spanning from

the top of the membrane to Fl, or right under a quarter of the length spanning towards the

top of F1. Furthermore, four amino acid insertions and deletions were successfully

accommodated throughout most of the dimerization and 8-binding domains of the b

subunit (manuscript in preparation). In contrast to the previously accepted role of the b

subunit as a rigid "stator", these observations suggest that the role of the b dimer is more

of a flexible or elastic structural feature during rotational catalysis. In the present