bl26, bl27, bl28, bl29, bl30, bl31, bl32, bl39, bl44, bl46, Or bl56 (Figure 1-5) (198, 200, 207).

Hydrodynamic evidence favors a folded structure for this domain of the b subunit as

opposed to the highly elongated structure of the remainder of the b subunit. Also, several

studies have shown that either a balal28-glu mutation, deletion of the last four residues, or

cold temperature dramatically decreased the sedimentation coefficient, by 23%,

suggesting that the F1-binding domain underwent a conformational change from a

globular structure to a less folded more extended conformation (192, 205, 208). The

mutation, balal28-glu, may have caused an electrostatic repulsion that would cause the two

b subunits to push apart. The carboxyl-terminal residues may form an amphipathic helix,

so the deletion would have disrupted essential interactions. And cold temperatures have

been shown to weaken hydrophobic interactions in proteins, suggesting the importance of

the hydrophobic amino acids in the folding of this domain (13). Dunn et al. suggested

that these observations implied that the carboxyl-terminus of the b subunit has a weakly

folded structure in which the hydrophobic amino acids are arranged to impart structural

stability and create hydrophobic patches on the surface (13). The folded conformation

appears to be required for the exposed hydrophobic patches to interact with Fl.

 Mutagenesis. Several mutant searches and site-directed mutagenesis studies have

been performed in the membrane-spanning domain of the b subunit. However, only a

single mutation, bgly9,asp, lOcated near the periplasmic side of the lipid bilayer, resulted in

a defective proton pore in an intact F1Fo ATP synthase complex (209). Second site

suppressors of this mutation have been found in the a(apro240-ala Of Opro240-leu) and c

(cala62-ser) Subunits that partially repaired the defect, indicating an interaction between the

b subunit and both the a and c subunits (210, 211). The membrane-spanning domain