brane helix is known to protonate and deprotonate during Htransport. Extensive cross-linking studies
with native bacterial F 0 indicated that subunits c (respectively subunits III) are arranged in a ring with a
diameter of about 5.5 nm (7 nm for CF 0 ) [69]. For CF 0 this is supported by electron and atomic force spec-
troscopy observations [86,87]. Helix 1 at all subunits is situated inside the ring, while helix 2 is outside
[88]. This packing supports the suggestion that the proton binding site is formed at the packed interface
of two units, with Asp61 at the front face of one subunit interacting with Ala24, Ile28, and Ala61 at the
back face of a second subunit [84].
The binding of the loop region to subunits andof CF 1 is proposed to force rotation of subunit
as proton transport drives rotation of the subunit III oligomer ring.
Subunit I is a chloroplast-encoded (atpF) protein with a molecular mass of about 21 kDa and one
transmembrane helix. Subunit II is similar to it but is a nuclear-encoded protein. It has a molecular mass
of 16 kDa and a single transmembrane helix. The organization of subunits I and II and their function are
also proposed on the basis of the resolved structure of the bacterial F 0 complex. Two subunits of b are pre-
sent of F 0 forming a dimer placed outside the c-oligomeric ring [84,89]. They play the role of a stator
holding 3 3 subunits of F 1 (CF 1 ) fixed to the stationary F 0 (CF 0 ) subunits as c 12 - ,subunits rotate as a
unit [84]. The cytoplasmic domain of b subunits (subunits I and II at CF 0 , respectively) binds to the sub-
unit of F 1 (CF 1 ) [90]. The interactions between subunit b and subunits andat the top of F 1 were
demonstrated by Rodger and Capaldi [91]. To reach the top of F 1 , subunit b is estimated to extend 11 nm
from the surface of the membrane [91]. Subunit b, like subunits I and II from CF 0 , is anchored in the mem-
brane via a single transmembrane helix at the N-terminal end [92].
Subunits I and II are bound to subunit IV probably without close contact with the subunit III oligomer
ring. As with bacterial ATP synthase, the major stator component tightly anchored in the membrane is
subunit IV (the bacterial homologue is subunit a). Subunit IV is a chloroplast-encoded protein (atpI) with
a molecular mass of 25 kDa and four transmembrane helices. It was proposed that subunit IV plays a cen-
tral role together with the subunit III ring of a functional conductance to protons of the assembled enzyme.
Nothing is known about the arrangement of the subunit IV transmembrane domains and the mechanisms
of its participation in proton transport in chloroplasts [69].
B. Supramolecular Organization of CF 1
The 3 : 3 : complex is the active site of ATP synthesis. The subunit is a chloroplast-encoded (atpA)
protein with molecular mass 55 kDa. It is entirely situated outside the membrane. It is accepted that the
subunit participates in nucleotide binding and has a regulatory role [69].
Thesubunit is also a chloroplast-encoded protein (atpB) with a mass of about 54 kDa. It is local-
ized outside the membrane and is a catalytic site of ATP synthase [69].
The subunit is a nuclear-encoded (atpC) protein with a molecular mass of 35 kDa. Its C- and N-ter-
minal regions are situated in the core of the 3 : 3 ring. The other part of the subunit protrudes below
the C-terminal domain of the andsubunits. Together with the subunit, the protruding portion of the
subunit is tightly bound to the surface-exposed loops of the subunit III oligomer. The role of the sub-
unit is as a transducer of rotational energy from the subunit III oligomer rotor [69,84].
The present arrangement of the enzyme stems from the study of a mitochondrial 3 : 3 subcomplex
[93]. The complex consists of a hexameric ring of alternating andsubunits that surrounds the -heli-
cal domain formed of both C- and N- terminal regions of the subunit. The top of the assembly, dis-
tal to the membrane, consists of a -barrel composed of the N-terminal portion, covering a central nu-
cleotide-binding domain, followed by the C-terminal, -helical domain. Three groups of
-heterodimers are thus formed, which can adopt three distinct nucleotide binding conformations cor-
responding to empty sites, ADP/Pi binding site, and ATP tight binding sites. These structural features sup-
port a rotatory mechanism wherein the central subunit rotates within the -hexamer, driving the en-
zyme through three successive configurations that are required for ATP synthesis/hydrolysis [69,84].
C. Function of ATP Synthase
The chloroplast ATP synthase generates ATP from ADP and inorganic phosphate using energy delivered
from a trans-thylakoid electrochemical proton gradient H[84]. At the membrane-embedded CF 0 por-
tion an energy-releasing proton transport takes place when the energy-consuming ATP synthesis occurs
PHOTOSYNTHETIC MEMBRANES IN HIGHER PLANTS 291