Abstract
Aquaporin 0 (AQP0) formerly known as membrane intrinsic protein (MIP), is expressed exclusively in the lens during terminal differentiation of fiber cells. AQP0 plays an important role not only in the regulation of water content but also in cell-to-cell adhesion of the lens fiber cells. We have investigated the thermal stress-induced structural alterations of detergent (octyl glucoside)-solubilized calf lens AQP0. The results show an increase in the amount of AQP0 that aggregated as the temperature increased from 40°C to 65°C. α-Crystallin, molecular chaperone abundantly present in the eye lens, completely prevented the AQP0 aggregation at a 1∶1 (weight/weight) ratio. Since α-crystallin consists of two gene products namely αA- and αB-crystallins, we have tested the recombinant proteins on their ability to prevent thermal-stress induced AQP0 aggregation. In contrast to the general observation made with other target proteins, αA-crystallin exhibited better chaperone-like activity towards AQP0 compared to αB-crystallin. Neither post-translational modifications (glycation) nor C-terminus truncation of AQP0 have any appreciable effect on its thermal aggregation properties. α-Crystallin offers similar protection against thermal aggregation as in the case of the unmodified AQP0, suggesting that αcrystallin may bind to either intracellular loops or other residues of AQP0 that become exposed during thermal stress. Far-UV circular dichroism studies indicated a loss of αhelical structures when AQP0 was subjected to temperatures above 45°C, and the presence of α-crystallin stabilized these secondary structures. We report here, for the first time, that α-crystallin protects AQP0 from thermal aggregation. Since stress-induced structural perturbations of AQP0 may affect the integrity of the lens, presence of the molecular chaperone, α-crystallin (particularly αA-crystallin) in close proximity to the lens membrane is physiologically relevant.
Highlights
Aquaporins (AQPs) are the family of homologous water and glycerol transporters expressed in eubacteria, archae, fungi, plants and animals [1]
Anecdotal evidence suggests that boiling lens membranes leads to aggregation of Aquaporin 0 (AQP0), this is the first systematic study to show it thermal-induced aggregation properties
Owing to its pivotal role in water transport and its involvement in cell-to-cell adhesion, understanding stress-induced aggregation of AQP0 is highly relevant in lens pathophysiology
Summary
Aquaporins (AQPs) are the family of homologous water and glycerol transporters expressed in eubacteria, archae, fungi, plants and animals [1]. In order to study the effect of glycation of AQP0 (a common post-translational modification in aging and in diabetes) on the chaperone function of a-crystallin, lens membranes were incubated without and with 1 M glucose for 4 days at 37uC as described earlier [33]. In order to determine the chaperone function of a–crystallin towards AQP0 during thermal-denaturation, bovine a-crystallin was added in the assays.
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