From ATP to PTP and Back: A Dual Function for the Mitochondrial ATP Synthase

Abstract

Mitochondria not only play a fundamental role in heart physiology but are also key effectors of dysfunction and death. This dual role assumes a new meaning after recent advances on the nature and regulation of the permeability transition pore, an inner membrane channel whose opening requires matrix Ca(2+) and is modulated by many effectors including reactive oxygen species, matrix cyclophilin D, Pi (inorganic phosphate), and matrix pH. The recent demonstration that the F-ATP synthase can reversibly undergo a Ca(2+)-dependent transition to form a channel that mediates the permeability transition opens new perspectives to the field. These findings demand a reassessment of the modifications of F-ATP synthase that take place in the heart under pathological conditions and of their potential role in determining the transition of F-ATP synthase from and energy-conserving into an energy-dissipating device.

Keywords: Ca(2+)-Mg(2+)-ATPase; mitochondria; permeability transition pore.

Figure 1. Structure of bovine F-ATP synthase

Left monomer, the F₁ and Fo sectors are highlighted. Right monomer, the F₁ and Fo subunits are shown. In the F₁ sector the front α and β subunits have been removed to reveal the F₁-rotor (central stalk). The F₁ α and β subunits are colored in red and yellow, respectively. The F₁-rotor γ, δ and ε subunits are colored in shades of blue, the peripheral stalk subunits b, d, F₆ and OSCP in shades of green, and the c-ring in purple. The remaining Fo subunits a, e, f, g, A6L, whose structure has not been defined yet, are located in the Fo subcomplex colored in light blue. The image (lateral view) has been built starting from the yeast dimer molecular model (PDB id. 4b2q) and superimposing the cryo-electron microscopy map of bovine F-ATP synthase (EMD id. EMD-2091). The fit of molecular models to cryo-electron microscopy map was performed using the program ADP_EM. The molecular model for bovine F-ATP synthase was obtained by superimposing the 3D structure of the bovine F₁-c-ring complex (PDB id. 2xnd) onto each corresponding monomer of the yeast dimer. The superposition was performed using the swiss pdb viewer routine Iterative magic fit. The lateral stalk was taken from the yeast dimer (PDB id. 4b2q) which contains the bovine subunits.

Figure 2. Interactors of F-ATP synthase and PTP formation

Reversible binding of CyPD and IF₁ to F-ATP synthase is shown, together with the putative region of channel formation (broken arrow). Factors promoting binding/release of interactors are also indicated.

Figure 3. Map of Mg²⁺/Ca²⁺ binding sites and Ca²⁺-dependent interactions in bovine F-ATP synthase

Left monomer, the subunits involved in Ca²⁺-dependent interactions are highlighted, i.e. subunits α and β, which interact with the matrix protein S100A1 (not shown in the picture), and the Fo region containing subunit e, which may interact with a hypothetical tropomyosin-like protein localized in the intermembrane space. Ca²⁺-regulatory sites located in the c-ring are also shown^,. Right monomer, the residues (T163, R189, E192, D256) of β subunit interacting with the catalytic metal ions are mapped onto the 3D structure of the bovine F₁-c-ring complex.

Figure 4. Map of Cys residues in bovine heart F-ATP synthase

Position of Cys residues on the specified subunits (red dots) is mapped onto the 3D structure of the bovine F₁-c-ring complex and of the bovine lateral stalk.

Figure 5. Posttranslational modifications of F-ATP synthase in the normal and failing heart

Left monomer, the residues involved in PTMs in normal heart are shown, i.e. complete trimethylation of cK43; oxidation of W12, W53 of subunit d; phosphorylation of αS184, αS419, γY52 and γS121, dS30. Right monomer, residues involved in PTMs associated with preconditioning or heart failure are shown, i.e. phosphorylation of βS56, βT57, βT212, βS213; nitration of d Y114 (numbering according to PDB id 4b2q) and hydroxylation of dK71. Subunit α is also highlighted, being the target of tyrosine nitration.