No abstract

Gap junctions formed by connexons and thin junctions formed by lens-specific aquaporin 0 (AQP0) mediate the tight packing of fibre cells necessary for lens transparency. Gap junctions conduct water, ions and metabolites between cells, whereas junctional AQP0 seems to be involved in cell adhesion. High-resolution atomic force microscopy (AFM) showed the supramolecular organization of these proteins in native lens core membranes, in which AQP0 forms two-dimensional arrays that are surrounded by densely packed gap junction channels. These junctional microdomains simultaneously provide adhesion and communication between fibre cells. The AFM topographs also showed that the extracellular loops of AQP0 in junctional microdomains adopt a conformation that closely resembles the structure of junctional AQP0, in which the water pore is thought to be closed. Finally, time-lapse AFM imaging provided insights into AQP0 array formation. This first high-resolution view of a multicomponent eukaryotic membrane shows how membrane proteins self-assemble into functional microdomains.

The ATP synthase is a nanometric rotary machine that uses a transmembrane electrochemical gradient to form ATP. The structures of most components of the ATP synthase are known, and their organization has been elucidated. However, the supramolecular assembly of ATP synthases in biological membranes remains unknown. Here we show with submolecular resolution the organization of ATP synthases in the yeast mitochondrial inner membranes. The atomic force microscopy images we have obtained show how these molecules form dimers with characteristic 15 nm distance between the axes of their rotors through stereospecific interactions of the membrane embedded portions of their stators. A different interaction surface is responsible for the formation of rows of dimers. Such an organization elucidates the role of the ATP synthase in mitochondrial morphology. Some dimers have a different morphology with 10 nm stalk-to-stalk distance, in line with ATP synthases that are accessible to IF 1 inhibition. Rotation torque compensation within ATP synthase dimers stabilizes the ATP synthase structure, in particular the stator-rotor interaction. Data analysis All image treatment and analysis of AFM topographs were performed using custom-written routines for the ImageJ image processing package (23,24) and IGOR PRO (Wavemetrics, Portland, OR).