Forum 10/01/2009

“The in situ arrangement of membrane protein complexes in mitochondria and chloroplasts, and its functional consequences”

Werner Kuhlbrandt – Max Planck Institute of Biophysics

We use electron cryo-tomography of isolated membranes, whole organelles or vitreous sections to examine the arrangement of large membrane protein complexes in mitochondria and chloroplasts. We found that the mitochondrial ATP synthase is arranged in long rows of dimers, which are always found at the position of highest membrane curvature of cristae membranes. Yeast mutants lacking the ATP synthase subunits thought to be responsible for dimer formation do not show this striking arrangement. Unlike the wt mitochondrial complex, the chloroplast ATP synthase is entirely monomeric, and confined to membrane regions of minimal curvature. We believe this to be directly related to differences in membrane potential and pH gradient in the two organelles. Visualizing the photosystem-II dimer in chloroplast membranes helps us to understand the molecular basis of lateral segregation into stacked and unstacked membrane regions. Finally, we are in the process of setting up a prototype instrument for in-focus phase contrast electron microscopy that will allow us to record tomograms and single particle images with higher contrast and, hopefully, resolution.