The protein that maintains the clarity of the lens in the human eye has been viewed at a dynamic, atomic-scale level using X-ray crystallography and electron microscopy.
Aquaporin 0 (AQP0), found only in the mammalian lens of the eye, maintains clarity in unique cells known as lens fibres. These constitute the majority of the lens, protected by an epithelial layer beneath the lens capsule membrane.
The fibres are packed in tightly with water and crystallin proteins to create a uniform medium that conveys light with a refractive power approaching that of glass.
The lens is also known as the aquula (“a little stream”)
Aquaporins, also known as water channels, are intrinsic membrane proteins that conduct water transport into and out of cells, through the hydrophobic lipid bilayer.
Faults in the AQP0 gene are known to cause congenital and possibly age-related cataracts. The study modelled the protein's interaction with calmodulin (CaM), which binds four calcium ions at a time, an activity that is modulated by a wide range of other signalling molecules.
The channel closes under the regulation of CaM, and with this work comes its mechanism. Rather than simply plug the channel, CaM grasps the open channel and forces it to shut.
Previous work had peered into this system using X-rays, but the technique is of limited use with such large groups of proteins, or for proteins in motion. The answer came by feeding in electron microscopy (EM) images, observing the complexed calmodulin.
In this study, we determined the pseudoatomic structure of full-length mammalian aquaporin-0 (AQP0, Bos taurus) in complex with CaM, using EM to elucidate how this signaling protein modulates water-channel function. Molecular dynamics and functional mutation studies reveal how CaM binding inhibits AQP0 water permeability by allosterically closing the cytoplasmic gate of AQP0. Our mechanistic model provides new insight, only possible in the context of the fully assembled channel, into how CaM regulates multimeric channels by facilitating cooperativity between adjacent subunits.
"Functional mutation studies" refers to deducing the most critical amino acids from the combined structural data, and selectively neutralising their charges and observing the resultant change (if any) in function.
AQP0 comprises 4 identical barrel-shaped units (monomers), bundled together side by side as shown above. The researchers found that in the presence of Ca2+, calmodulin binds to one unit and then another, as if grabbing the "reins of the channel", making the channel twist slightly, causing just a few amino acids within each unit to slide into the channel’s core and block the flow of water.
Calmodulin is found throughout the body — regulating channels vital for nerve firing, muscle contraction, and the rhythmic beating of the heart. This study notably provides "the first complete structural model for any full-length membrane channel in complex with CaM", and the authors seem eager to investigate how widespread this mechanism may be.
For AQP0, each monomer is itself a fully active water pore. The tetramerisation seems only necessary then as a "scaffold" for the cooperative binding of regulatory proteins such as calmodulin (which binds 2 such subunits at a time, and simultaneously).
This 2:1 stoichiometry is seen in other channels, such as the NMDA receptor's two NR1 subunits, at which co-agonist glycine is bound in conjunction with the neurotransmitter glutamate at NR2 subunits.
Although there is currently no structural information on this complex, it is conceivable that this is another case in which the stoichiometry provides the mechanical force used to allosterically drive channel closure.
Reichow et al (2013) Allosteric mechanism of water-channel gating by Ca2+–calmodulin. Nature Structural and Molecular Biology 20(9): 1085–1092