aquaporin - junctional and nonjunctional

AQP0 Aquaporin Membrane Proteins

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"AQP0 serves a dual function in the lens, acting both as a water channel and as an adhesion molecule. During the differentiation of fibre cells, and as they grow older and become buried more deeply in the lens, AQP0 is cleaved at both termini. This processing seems to be the trigger for junction formation." (Gonen et. al 2005)

The opening screen shows a meshribbon model of the overlayed junctional and non-junctional AQP0 protein.

The green is the non-junctional and the red is the junctional.

In some of the following models, temperature coloring emphasizes areas of less conformational constraint in red.

For more information see Biochemistry Online: Chapter 6A: Passive and Facilitated Diffusion

Junctional Wire Frame
Junctional Wire Frame (Temperature Coloring)
Non-junctional Wire Frame
Non-junctional Wire Frame (Temperature Coloring)
Junctional Cartoon
Non-junctional Cartoon

AQP0 aquaporin pores are mainly lined with alpha helices, which can be seen clearly using the cartoon representation.

Junctional Cartoon and Wireframe
Non-Junctional Cartoon and Wireframe
Junctional spacefill: sphere 0A
Non-junctional spacefill: sphere 0A
Junctional solvent accessible surface: dots
Non-junctional solvent accessible surface: dots

The next surface displays are solid and require calculations with some time delays

Junctional AQP0 model of solvent accessible surface: solid (1.2 A) - surface at center of 1.2 sphere (sasurface)
Non-juntional AQP0 model of solvent accessible surface: solid (1.2 A) - surface at center of 1.2 sphere (sasurface)
Junctional AQPO solvent excluded molecular surface - (solid 1.4 A) - surface at contact with sphere (molecular)
Non-junctional AQPO solvent excluded molecular surface - (solid 1.4 A) - surface at contact with sphere (molecular)

The following contain isosurface modeling of AQP0 in both junctional and non-junctional conformations. The green areas around the center contain residues that are mostly non-polar as they must interact with the lipid membrane.

Both juntional and non-junctional display very similar electrostatic potential distributions.

Map of Electrostatic potential from non-juctional AQPO with wireframe rendering

It turns out formation of the AQP0 tetrameter is mediated partially through the Trp 34 and Pro 38 residues. Comparison of the structure of junctional AQP0 with that of non-junctional AQP0 indicates that cleavage of the two cytoplasmic termini might lead to a conformational change, "eliminating steric hindrance from Trp 34 and allowing Pro 38 to stabilize the junctional interaction [with other AQP0 monomers]." (Gonen et. al 2005)

"Formation of the junction also seems to correlate with changes in the side-chain positions of residues lining the pore, most importantly in Met 176, resulting in substantial constriction. Three water molecules are trapped in the centre of the water pathway, too far apart from each other to be linked by hydrogen bonds." (Gonen et. al 2005)

The direction of Met 176 determines the flow of waters through the pore in the monomer. The differences can be seen in both models:

Junctional (closed) AQP0 Cartoon with conformation altering residues: Trp 34 (Red) , Pro 38 (Purple),Met 176 (Green)
Non-Junctional (open) AQP0 Cartoon with conformation altering residues: Trp 34 (Red), Pro 38 (Purple), Met 176 (Green)

Source:

Gonen, T.; Cheng, Y.; Sliz, P.; Hiroaki, Y.; Fujiyoshi, Y.; Harrison, S; Walz, T. "Lipid–protein interactions in double-layered two-dimensional AQP0 crystals". Nature. 2005 Dec 1; 438(7068); p. 633-8.