ATP synthase--the structure of the stator stalk

Abstract

ATP synthase synthesizes ATP from ADP and inorganic phosphate using a unique rotary mechanism whereby two subcomplexes move relative to each other, powered by a proton or sodium gradient. The non-rotating parts of the machinery are held together by the "stator stalk". The recent resolution of the structure of a major portion of the stator stalk of mitochondrial ATP synthase represents an important step towards a structural model for the ATP synthase holoenzyme.

1

The rotary mechanism of ATP synthase. A model of the E. coli enzyme is shown (taken from Ref. [14], with permission). The portions of the enzyme for which a high-resolution structural model was available are shown in surface representation. The individual structural models (α₃β₃γε [23], N-terminal domain of δ bound to N-terminus of α [13], ac_n [24], membrane portion of b [8], “dimerization domain” of b [9]) were docked by eye. Structural data for parts of a and b as well as the C-terminal domain of δ were lacking. ATP synthase can be separated into a membrane-standing subcomplex, F_o, which, in E. coli, consists of subunits ab₂c_n (brown, pink, and alternating cyan/light green, respectively) and a soluble subcomplex, F₁, consisting of subunits α₃β₃γδε (red, yellow, blue, orange, and green, respectively). A newer, more mechanically-based division differentiates between the “rotor” (in E. coli, subunits γεc_n) and the “stator” (subunits α₃β₃δab₂). The dashed blue line indicates the part of γ hidden from view in the center of the α₃β₃ ring. Flow of protons, down a gradient, through channels in the a subunit at the interface to the c_n ring drives the rotor, whose movement results in synthesis of ATP from ADP and P_i in the catalytic nucleotide binding sites on the β subunits and release of product ATP from these sites. The enzyme can also run in reverse, hydrolyzing ATP to generate a proton gradient. In the mitochondrial enzyme, the b dimer is replaced by a single b subunit plus subunits d and F₆. The b/d/F₆ subcomplex whose structure was resolved by Kane Dickson et al. [10] corresponds to the hatched area. The mitochondrial counterparts of E. coli subunits δ and ε are termed OSCP (“oligomycin-sensitivity conferring protein”) and δ, respectively. Figures 1 and 2A were created with PyMOL (pymol.sourceforge.net).

2

Structure and localization of the stator stalk of mitochondrial ATP synthase. (A) Structure of the b/d/F₆ subcomplex [10]. Subunit b is depicted in red, subunit d in green, subunit F₆ in blue. For each subunit, the N-terminus of the portion included in the crystallized subcomplex is marked. Black arrows indicate where some of the subunit fragments not included in the crystallized subcomplex might be accomodated. b_C, C-terminus of subunit b; d_C, C-terminus of subunit d; OSCP_C, C-terminus of subunit OSCP (OSCP corresponds to the bacterial subunit δ in Fig. 1). (B) Localization of the stator stalk. The b/d/F₆ subcomplex [10] (red/green/blue) was docked by eye to the F₁c₁₀subcomplex [11] (grey), using RasMol (www.bernstein-plus-sons.com), and both were modeled into a model of mitochondrial ATP synthase derived by cryo-electron microscopy (orange; after [12], with permission). The approximate location of subunit a, the N-terminus of b (“b_N”), and the N-terminus of OSCP (“OSCP_N”) is indicated. Mitochondrial ATP synthase contains a number of supernumerary proteins in the transmembrane region (marked with an asterisk) that are absent in the bacterial and chloroplast enzymes.