Structure of aquaporin-0 arrays in sphingomyelin/cholesterol membranes and implications for lipid rafts.

The tetrameric water channel aquaporin-0 (AQP0) forms square arrays in lens membranes through an as yet unknown mechanism. Lens membranes are enriched in cholesterol and sphingomyelin, suggesting that these raft lipids may play a role in AQP0 array formation. We produced two-dimensional crystals of AQP0 in sphingomyelin membranes with low and high cholesterol content and determined their structures by electron crystallography. At the higher cholesterol concentration, cholesterol associates with AQP0 and shifts lipids in the cytoplasmic leaflet away from the bilayer center, increasing the hydrophobic thickness of the annular lipid shell. AQP0 tetramers may thus cluster to mitigate the resulting hydrophobic mismatch. Moreover, neighboring AQP0 tetramers sandwich a cholesterol in the center of the membrane. Since each tetramer interacts with four such 'glue' cholesterols, avidity effects may stabilize larger arrays. The principles proposed to drive AQP0 array formation could also underlie protein clustering in lipid rafts.