Aquaporin-0 interacts with the FERM domain of ezrin/radixin/moesin proteins in the ocular lens

The lens of the eye is a transparent tissue composed of an anterior monolayer of epithelial cells and the underlying highly differentiated and elongated lens fiber cells that form the bulk of the organ. Lens transparency is critical to its function of focusing of light onto the retina, and this transparency is achieved and maintained by precise cell–cell interactions. Lens fiber cells are closely packed together with extensive contact between adjacent cells forming cell-to-cell adhesive complexes, the cortex adhaerens.1 Disruption of fiber cell packing can lead to light scattering and cataract.2 Aquaporin 0 (AQP0), the major intrinsic membrane protein of the lens, is the most abundant membrane protein in the lens, constituting 50% to 60% of the plasma membrane protein,3 and it plays important roles in the maintenance of lens transparency and homeostasis. Mutation in and knockout of the AQP0 gene results in lens cataract.4–7 AQP0 has been suggested to perform multiple roles in the lens, including functioning as a water channel4,8,9 and as a cell adhesion molecule.10–15 AQP0 has six transmembrane domains, with its N terminus and C terminus located in the cytoplasm. The cytoplasmic C terminus of AQP0 is predicted to be functionally significant because it contains sites of modifications and protein–protein interactions. The C terminus of AQP0 contains several phosphorylation sites16,17 and a calmodulin interaction site,18 all of which have been reported to impact AQP0 permeability.19,20 The C terminus of AQP0 has also been reported to interact with the cytoskeletal proteins filensin and CP49, suggesting a role for AQP0 in establishing and/or maintaining fiber cell shape.21 The ezrin/radixin/moesin (ERM) protein family is part of the band 4.1 super family, which links actin filaments to the plasma membrane.22–24 Specific ERM proteins are involved in many important roles, such as cell signaling, stabilizing adhesion junctions, the maintenance of cell shape, villar organization in the gut, and the light-regulated maintenance of photoreceptors, etc.25 ERM protein members all share a common homologous ∼300–amino acid domain called the FERM domain (F for 4.1 protein, E for ezrin, R for radixin, and M for moesin).26 ERM proteins bind with membrane proteins such as, CD44, intracellular adhesion molecule-1 and −3, P-selectin glycoprotein ligand-1, and others through an N-terminal FERM domain and connect to F-actin through a C-terminal domain. The presence of ERM proteins in lens fiber cells has been shown1,27–29; however, the function of ERM proteins in lens fiber cells has not been extensively studied. Ezrin was identified as one of the components of a novel cell–cell junction system of lens fiber cells, which contains ezrin, periplakin, periaxin, and desmoyokin (EPPD complex).1 It is unclear how the EPPD complex is linked to the plasma membrane, because the transmembrane protein in this complex has not been determined. In this article, we report a direct interaction between the FERM domain of ezrin and AQP0. The results indicate that AQP0 is a candidate membrane protein attachment site of the EPPD complex.