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(25) DYNAMICS OF MITOCHONDRIAL INNER MEMBRANE FOLDING INFERRED FROM EM TOMOGRAPHY C.A. Mannella1, K.
Buttle1, C. Hsieh1, D. Pfeiffer2, P. Bradshaw2
and M. Marko1
Three-dimensional images provided by electron tomography of conventionally fixed and embedded mitochondria indicate that the standard model for mitochondrial structure (inferred years ago from 2D images) is incorrect. Mitochondrial cristae generally have restricted openings to the outer (intermembrane) compartment. In mammalian mitochondria, the cristae usually connect through narrow, sometimes very long tubular regions to the peripheral surface of the inner membrane and to each other [1-3]. Modeling studies suggest that this design feature may lead to formation of lateral gradients of important metabolites (like ADP) within the cristae (see poster by Moraru et al.). Thus, changes in inner membrane shape may regulate ATP production. Inner membranes of highly condensed yeast mitochondria form a single, highly curved surface with no tubular regions. During large-scale swelling (triggered by activation of a permeability transition), the inner membrane simply unfolds, with no tubular regions apparent. However, subsequent osmotic shrinkage leads to the occurrence of either mammalian-like cristae, with short tubular junctions, or vesicular cristae, having no connections to the external compartment. The latter can be expected to have reduced bioenergetic function, since the internal compartments cannot communicate by simple diffusion with the outside. Vesicular cristae have been observed in one other case, a cytopathy tentatively characterized as a deficiency in adenine nucleotide transporter [4]. Recently, tomograms have been obtained of rat liver and fungal mitochondria that have been quick-frozen in suspension and embedded in vitreous ice, without fixatives or stain [5]. These 3D images of native, frozen-hydrated mitochondria confirm the tubular nature of cristae and strongly suggest that larger compartments form by fusion of tubes. Interestingly, cristae in both types of mitochondria exhibit tubular junctions as well as wider, slot-like openings. We are exploring the possibility that surface-tension-induced compression (20-50%) of the organelles prior to freezing may be sufficient to cause fusion of tubular cristae. Tomography development and application at Wadsworth’s Resource for the Visualization of Biological Complexity is supported by NIH grant RR01219. [1] Mannella, C. et al.
(1994) Microscopy Res. Tech. 27:278
For further information contact...Carmen Mannella: carmen@wadsworth.org |
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