Regulation of Mitochondrial Outer and Inner Membrane Fusion Coupling

Abstract

Mitochondrial fusion is an essential process for maintaining cell health that entails the sequential mergers of outer membrane (OMM) and inner membrane (IMM). While IMM fusion is not obligatory following fusion of the OMM, it most often follows in a matter of seconds. Little is known about the coordination of this close coupling. Using photactivatable fluorescent proteins and time-lapse confocal microscopy we have been able to visualize and quantify OMM-IMM fusion coupling in vivo under a variety of conditions to discover regulation by two inter-related factors. First, in cells deprived of oxidative substrate, reintroduction of glutamine significantly increased the efficiency of coupling as measured by the distribution of coupling times and OMM-only fusion events. This enhancement correlates with mitochondrial ATP levels measured by a matrix-targeted genetically encoded reporter, and supported by a strong decoupling effect observed following oligomycin treatment. Second, cells treated with carefully calibrated levels of the ionophores valinomycin or nigericin show increased and decreased coupling efficiency, respectively. We hypothesize this effect is due to matrix volume dilation and constriction that affects the physical interaction of the OMM and IMM.