Aquaglyceroporins: channel proteins with a conserved core, multiple functions, and variable surfaces.

Area of Science:

• Membrane Biology
• Structural Biology
• Biophysics

Background:

• Membrane channels are crucial for osmoregulation across all domains of life.
• Aquaporin-1 (AQP1) was the first water channel functionally characterized in human erythrocytes.
• Phylogenetic analysis identified two major subfamilies: aquaporins (AQPs) and glycerol facilitators (GLPs).

Purpose of the Study:

• To elucidate the structure and function of aquaporins and glycerol facilitators.
• To understand the molecular mechanisms of water and small nonionic solute transport.
• To provide atomic-level insights into channel selectivity and proton exclusion.

Main Methods:

• Expression of AQP1 in Xenopus oocytes to demonstrate water conductivity.
• Phylogenetic analyses to classify AQP and GLP subfamilies.
• Sequence-based structure prediction and analysis of strategic residues.
• Atomic force microscopy to map AQP surface topography.
• Electron cryomicroscopy (cryo-EM) for 3D structure determination of AQP1 and GlpF.
• X-ray crystallography for high-resolution structure of GlpF.

Main Results:

• AQP1 was functionally validated as a water channel.
• Structural models predicted six membrane-spanning helices for AQPs.
• Surface topography revealed distinct features correlating with loop structures.
• Cryo-EM yielded the first atomic model of AQP1 (3.8 Å), showing water permeation and proton blockage.
• GlpF structure (6.9 Å cryo-EM, 2.2 Å X-ray) confirmed helical similarity to AQP1.
• Homology modeling indicated conserved channel regions in aquaglyceroporins.

Conclusions:

• Structural and functional studies provide atomic insights into AQP and GLP mechanisms.
• The determined structures reveal how these channels facilitate transport while maintaining selectivity.
• Conserved structural elements suggest a common evolutionary origin and functional principle for aquaglyceroporins.