Determination of protein mobility in mitochondrial membranes of living cells

Molecular mobility in membranes of intracellular organelles is poorly understood, due to the lack of experimental tools applicable for a great diversity of shapes and sizes such organelles can acquire. Determinations of diffusion within the plasma membrane or cytosol are based mostly on the assumption of an infinite flat space, not valid for curved membranes of smaller organelles. Here we extend the application of FRAP to mitochondria of living cells by application of numerical analysis to data collected from a small region inside a single organelle. The spatiotemporal pattern of light pulses generated by the laser scanning microscope during the measurement is reconstructed in silico and consequently the values of diffusion parameters best suited to the particular organelle are found. The mobility of the outer membrane proteins hFis and Tom7, as well as oxidative phosphorylation complexes COX and F1F0 ATPase located in the inner membrane is analyzed in detail. Several alternative models of diffusivity applied to these proteins provide insight into the mechanisms determining the rate of motion in each of the membranes. Tom7 and hFis move along the mitochondrial axis in the outer membrane with similar diffusion coefficients (D = 0.7 μm2/s and 0.6 μm2/s respectively) and equal immobile fraction (7%). The notably slower motion of the inner membrane proteins is best represented by a dual-component model with approximately equal partitioning of the fractions (F1F0 ATPase: 0.4 μm2/s and 0.0005 μm2/s; COX: 0.3 μm2/s and 0.007 μm2/s). The mobility patterns specific for the membranes of this organelle are unambiguously distinguishable from those of the plasma membrane or artificial lipid environments: The parameters of mitochondrial proteins indicate a distinct set of factors responsible for their diffusion characteristics.

Research Highlights
► Protein mobility in membranes of mitochondria is different from other biomembranes.
► Proteins belonging to the same membrane display similar diffusivities.
► Diffusion in the inner membrane is much slower than in the outer membrane.