Water channel aquaporin-0
Ovis aries (sheep)
Aquaporins, water channels, are essential membrane proteins for the water transport, which can be permeated waters selectively. The lens-specific aquaporin-0 (AQP0) forms the two-layered channels connected with the membrane junctions, differently from the other aquaporins. The water permeability of AQP0 at mildly acidic conditions (pH 6.5) is twice higher than that at neutral pH. It is therefore considered that AQP0 water pores are gated.
The structure of AQP0 at pH 6.0 has been determined. Four adjacent molecules form a channel layer, and along with the symmetrical four molecules forming an opposite layer, the two-layered AQP0 are formed. The two layers are connected with the junctions formed by three important interactions. The most important interaction of them involves eight Pro38s of the loop A from the eight molecules of two layers. The others relate to the loop C; the Pro-Pro motif (Pro109 and Pro110) of the loop C interacts with that of the opposite layer, and Pro123 also interacts with Arg113 of the opposite layer. Because the loop C in AQP0 is shorter than that in the other aquaporins, the AQP0 loop C is advantageous for the formation of a tight junction. The constriction sites at both ends of the water pore are very longer than the aquaporin-1 (AQP1) constriction sites, and moreover, the pore-diameters of both sites in AQP0 are only 2 angstrom. It is thus unlikely that waters permeate through the narrow pore in AQP0. However the following structural features of AQP0 suggest that this structure of AQP0 is in the gate-closed conformation, and waters can permeate through the pore when the gate is opened. (1) Because Tyr149 near one end of the pore indicates the high mobility, it may induce an open and close of the pore gate. (2) Arg187 in the constriction site is stabilized by the hydrogen bond with Asn119 of the loop C. The junction formations involving the other residues of the loop C move Asn119 into a position where it can stabilize the alternative conformation of Arg187. The junction formations may namely cause the conformational changes of the pore. (3) Moreover, the protonation and deprotonation of His40 and His66 near both ends of the pore may affect the water permeability. It has therefore been proposed that the junction formations induce the closure of the AQP0 water pore.
Protein Data Bank (PDB)
Gonen, T. Sliz, P. Kistler, J. Cheng, Y. Walz, T.; "Aquaporin-0 membrane junctions reveal the structure of a closed water pore"; nature; (2004) 429:193-197 PubMed:15141214.
author: Yuko Tsuchiya