a-helix (137). The amino-terminal domain, 61-105, consisted of a six a-helix bundle with

the dimensions 45 x 20 x 30 A+. Helices 1 (64-20) and 2 (624-38), and helices 5 (670-81) and 6

(688-104) Organized into V-shapes that intercalated to form a core. Helices 3 (641-47) and 4

(653-64) were packed compactly against this four-helix core. Following this globularly

packed domain there was a loop region followed by a seventh a-helix (6118-129).

Comparison of the structural data for the intact 8 subunit against that of the 61-134

fragment illustrated the same structure for residues 1-104, but the spectral shift of

residues 105-134 was very different. It was possible that the carboxyl-terminal 42

residues missing from the 61-134 fragment affects this region of the 6 subunit.

 8 subunit topology. Taken together with biochemical and immunological data, the

structure revealed by NMR revealed that the 6 subunit consists of two domains, an amino

terminal domain, 61-104, and a carboxyl-terminal domain, 6105-177. Under oxidizing

conditions, two native cysteines present in the amino- and carboxyl-terminal domains of

the intact 8 subunit, Scys64 and Scysl40, TOSpectively, formed a disulfide bond. Furthermore,

NMR data indicated some NOE's between the carboxyl-terminal a-helix and the amino-

terminal domain. The data indicated that there is probably a close interaction between

the amino- and carboxyl-terminal domains of the intact 8 subunit.

 Proteinase accessibility and immunological analysis were used to examine the

topology of the 6 subunit (89). The 6 subunit was susceptible to trypsin digestion at the

carboxyl-terminal 20 residues in isolated Fl, but not in intact F1Fo ATP synthase,

indicating a protection of the amino-terminal region by Fl. Deletion analysis of the

carboxyl-terminal region also implied the importance of the 6 subunit in binding Fl to Fo.