Journal
JOURNAL OF MOLECULAR BIOLOGY
Volume 300, Issue 4, Pages 779-789Publisher
ACADEMIC PRESS LTD
DOI: 10.1006/jmbi.2000.3920
Keywords
atomic force microscopy; aquaporins; major intrinsic protein; sidedness; two-dimensional crystals
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The lens major intrinsic protein (MTP, AQP0) is known to function as a water and solute channel. However, MIP has also been reported to occur in close membrane contacts between lens fiber cells, indicating that it has adhesive properties in addition to its channel function. Using atomic force and cryo-electron microscopy we document that crystalline sheets reconstituted from purified ovine lens MIP mostly consisted of two layers. MIP lattices in the apposing membranes were in precise register, and determination of the membrane sidedness demonstrated that MIP molecules bound to each other via their extracellular surfaces. The surface structure of the latter was resolved to 0.61 nm and revealed two protruding domains providing a tight tongue-and-groove'' fit between apposing MIP molecules. Cryo-electron crystallography produced a projection map at 0.69 nm resolution with a mirror symmetry axis at 45 degrees to the lattice which was consistent with the double-layered nature of the reconstituted sheets. These data strongly suggest an adhesive function of MIP, and strengthen the view that MIP serves dual roles in the lens. (C) 2000 Academic Press.
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