acids), to the carboxyl-terminus of the y subunit without interrupting function of the

enzyme complex. GFP forms a rigid, stable structure with the dimensions 24 A+ wide and

48 A+ high (188). This study indicated that the putative cap structure could not possibly

occupy the entire dimple atop Fl. More recent evidence has suggested that the 6 subunit

may actually be positioned slightly to the side of F1 in association with only a single a

subunit (Figure 1-2) (100, 141, 142, 189).

The bsubunit

 The b subunit is required for the normal assembly and function of F1Fo ATP

synthase (190). The E. coli F1Fo ATP synthase has two identical b subunits, which form

a homodimer, that are the product of a single gene (Figure 1-2). It is an elongated

amphipathic polypeptide that crosses the membrane one time at its amino-terminus and

has an extensive hydrophilic carboxyl-terminal domain. This pattern is characteristic of

b-type subunits of ATP synthases, although the mitochondrial b has two consecutive

membrane-spanning segments at the amino-terminus (191). Most ATP synthase b-type

subunits consist of between 150 and 170 amino acid residues. The E. coli b subunit,

encoded by the uncF gene, consists of 156 amino acid residues and has a deduced

molecular weight of 17, 265 Da (Figure 1-5).

 Domains and Structure. Currently there is no high-resolution structure of the

entire b subunit. Several factors probably contribute to the difficulty of structural

analysis. The b dimer is a thin, highly extended, mostly a-helical structure, its

dimerization is comparatively weak and reversible (192), and it displays evidence of

flexibility (193-195). This has led to alternative low-resolution approaches to study the

structure of the b dimer such as circular dichroism (CD) spectroscopy, deletion analysis,