analytical ultracentrifugation, chemical crosslinking, and the analysis of tendencies for

disulfide bond formation. CD spectroscopy analysis has predicted the secondary

structure of the b subunit to be approximately 80% ot-helical with about 14% p-turn

conformation (196).

 Although there is no high-resolution structure of the intact Fo sector, an abundance

of evidence suggests the necessity of the b subunit to exist in the dimeric state. The

hydrophilic region of b, consisting of residues b24-156 (alSo known as bsol), has been

expressed and shown to form highly extended dimers capable of binding to F1-ATPase in

solution (197). Sedimentation equilibrium ultracentrifugation gives a molecular weight

value of about 30,000 Da for bsoi, consistent with a dimer of two 15,000 Da bsol

monomers (13). The existence of the dimeric state of the b subunits was confirmed by

covalently cross-linking the two b subunits in the complex and verifying the activity of

the enzyme (198). Furthermore, the ability of b to bind to Fl was discovered to be

directly proportional to the ability of b to form dimers, suggesting the necessity of the b

dimer formation before the binding of Fl to the complex (199).

 The dimerization of the b subunit has been shown to be relatively weak and

reversible. The monomeric and dimeric forms of bsol were shown to exist in a dynamic

equilibrium and the dimer was converted to the monomeric state at 400C (192). This

same melting characteristic was observed with CD spectroscopy (200). Furthermore, the

similar traits were observed in photosynthetic organisms, which encode two different b-

type subunits, b and b '(13). When the cytoplasmic regions of the b and b subunitss from

the cyanobacterium Synechocystis were expressed individually, the polypeptides were

found to only exist in the monomeric state. However, when they were mixed together,