CHAPTER 3
 GENETIC COMPLEMENTATION BETWEEN MUTANT b SUBUNITS IN F1Fo ATP
 SYNTHASE

 Introduction

 In the F1Fo ATP synthase enzyme complex, the peripheral stalk consists of a

parallel dimer of identical b subunits. Dimerization of the b subunits is thought to be an

early event necessary for enzyme assembly and function (199). The two b subunits exist

in an extended a-helical conformation, spanning from the periplasmic side of the

membrane to near the top of F1. However, due to the asymmetric nature of the enzyme

complex the two b subunits cannot participate in identical protein-protein interactions

with the other subunits. In the amino-terminal membrane spanning region, the peripheral

stalk contacts a single a subunit. Similarly, at the carboxyl end of the peripheral stalk,

the two b subunits interact with a single 6 subunit. The b dimer has been studied by a

variety of traditional biochemical approaches such as CD spectroscopy, cross-linking,

and sedimentation experiments (30, 166, 192, 196, 197, 203, 283). Limited structural

information is available from studies of polypeptides modeling functionally defined

domains of the b subunit. Dmitriev et al. studied the amino-terminal membrane spanning

domain by NMR using a 34 amino acid polypeptide, revealing an a-helix with a 200 bend

at pro-27 and pro-28 (136). A systematic mutagenesis approach supported the model

described in the NMR paper suggesting that the extreme amino-termini of the two b

subunits were in close contact and then the subunits flare apart as they traverse the

membrane (136, 202). X-ray crystallography of a model polypeptide reflecting residues