IF 1 : setting the pace of the F 1 F o -ATP synthase Michelangelo Campanella 1, 2 , Nadeene Parker 1 , Choon Hong Tan 1 , Andrew M. Hall 1 and Michael R. Duchen 1 1 Department of Cell and Developmental Biology, Mitochondrial Biology Group, University College London, London, WC1E 6BT, UK 2 Royal Veterinary College, University of London, London, NW1 0TU, UK When mitochondrial function is compromised and the mitochondrial membrane potential (Dc m ) falls below a threshold, the F 1 F o -ATP synthase can reverse, hydrolysing ATP to pump protons out of the mitochon- drial matrix. Although this activity can deplete ATP and precipitate cell death, it is limited by the mitochondrial protein IF 1 , an endogenous F 1 F o -ATPase inhibitor. IF 1 , therefore, preserves ATP at the expense of Dc m . Despite a wealth of detailed knowledge on the bio- chemistry of the interaction of IF 1 and the F 1 F o -ATPase, little is known about its physiological activity. Emer- ging research suggests that IF 1 has a wider ranging impact on mitochondrial structure and function than previously thought. Mitochondria as ATP consumers In most Biochemistry text books, mitochondria are described as ‘the powerhouse of the cell’. The bulk of ATP in most mammalian cells comes from mitochondrial oxidative phosphorylation [1]. The reliable supply of ATP is fundamental to cell function; ATP is required for all the work that cells do: for muscle contraction, cell migration, secretion, and the maintenance of the ion gradients that underlie membrane excitability. Although ATP availabil- ity is a basic cellular requirement, the role of mitochondria in cell physiology extends far beyond even this. Mitochon- dria have essential roles in calcium (Ca 2+ ) homeostasis (for a recent review see Ref. [2]), in free radical signalling [3], and they act as gatekeepers of cell death by harbouring both pro- and anti-apoptotic proteins [4]. When a system is so fundamental to cell life and death, it follows inevitably that, when it is compromised, resultant impairment of cell and tissue function will manifest as disease. Indeed, dis- ordered mitochondrial function has been implicated in the pathogenesis of an array of major human diseases [5].A comprehensive understanding of the mechanisms that govern mitochondrial homeostasis and that dictate responses to altered mitochondrial homeostasis is clearly essential if we are to develop rational approaches to mana- ging these diseases. Central to mitochondrial function is an electrochemical proton gradient across the mitochondrial inner membrane that is established by the proton pumping activity of the respiratory chain (Figure 1). The proton gradient establishes a proton-motive force, which has two components: a pH differential and an electrical membrane potential (Dc m ). The pH component of mitochondrial proton motive force is small relative to the membrane potential and, hence, it is the latter that provides the predominant driving force for mitochondrial transport, ADP phosphorylation, Ca 2+ accumulation and the import of nuclear-encoded, mitochondrial-localized proteins. Impaired mitochondrial function resulting from a variety of different mechanisms will usually cause a decrease in Dc m as a common endpoint. These include limited sub- strate or oxygen availability such as occurs in ischaemia (e.g. in a stroke or heart attack); genetic or acquired defects in respiratory chain activity (due to mutations or to oxi- dative or nitrosative damage to mitochondrial respiratory proteins); or a leak of protons back into the mitochondrial matrix (through opening of the mitochondrial permeability transition pore [mPTP] [3,6] or the activation of uncoupling proteins; see Ref. [7] for a review). The F 1 F o -ATP synthase is the enzyme complex respon- sible for ATP synthesis driven by oxidative phosphoryl- ation [8,9]. The complex is an ancestral proton- translocating ATPase, a molecular motor that normally operates as an ATP synthase in the mitochondrial inner membrane in which ADP phosphorylation is driven by the movement of protons down the electrochemical potential gradient established by respiration (Figure 1a[i]; animations at: www.mrc-mbu.cam.ac.uk/research/atp- synthase). The directionality of the enzyme is dictated by the balance between the bioenergetic parameters of free energy available from the phosphorylation potential and from Dc m . In normally respiring mitochondria, the removal of ATP by the adenine nucleotide translocase (ANT) ensures that the intramitochondrial phosphoryl- ation potential is held relatively low while Dc m is high, (estimated at between 150 and 180 mV negative to the cytosol), favouring ADP phosphorylation (i.e. ATP syn- thesis). However, when mitochondrial homeostasis is com- promised, the situation can reverse. A decrease in Dc m accompanied by an increase in the phosphorylation poten- tial as glycolysis is upregulated (termed the Pasteur effect [10]; see also Ref. [11]) together with reversal of the ANT, which imports glycolytic ATP, will favour ATP hydrolysis. Therefore, during mitochondrial dysfunction, the F 1 F o - ATPase can run ‘backwards’, acting as an ATP-consuming proton pump (Figure 1aii). Review Corresponding author: Duchen, M.R. (m.duchen@ucl.ac.uk) 0968-0004/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.tibs.2009.03.006 Available online 24 June 2009 343