Electron crystallography of aquaporins

Abstract

Aquaporins are a family of ubiquitous membrane proteins that form a pore for the permeation of water. Both electron and X-ray crystallography played major roles in determining the atomic structures of a number of aquaporins. This review focuses on electron crystallography, and its contribution to the field of aquaporin biology. We briefly discuss electron crystallography and the two-dimensional crystallization process. We describe features of aquaporins common to both electron and X-ray crystallographic structures; as well as some structural insights unique to electron crystallography, including aquaporin junction formation and lipid-protein interactions.

Figure 1

Structure of aquaporin-1. A: Aquaporins are tetramers but each monomer forms a functional water pore (asterisk). B: Side view of an aquaporin-1 monomer. Six transmembrane helices pack against one another forming a barrel-like structure with the hydrophilic water pore at the center (green). The ar/R constriction site, or selectivity filter, is shown in red (Protein Data Bank accession No. 1FQY).

Figure 2

Structure of the aquaporin-0 mediated membrane junction. A: Side view of the aquaporin-0 junction. Two opposing tetramers interact in a head-to-head fashion. B: A major adhesive contact is mediated by proline 38, forming a rosette-like structure at the very center of the stacked tetramers (Protein Data Bank accession No. 2B6O).

Figure 3

Double layered aquaporin-4 2D crystals. A and B: Side and top views of the double-layered 2D crystals of aquaporin-4, respectively. Each aquaporin-4 tetramer contacts four tetramers from the opposing layer (Protein Data Bank accession No. 2D57).

Figure 4

Lipid-protein interactions in aquaporin-0 double-layered 2D crystals. A: Lipids (blue) pack tightly around the aquaporin-0 tetramer (grey). Lipid acyl chains fit snuggly into irregularities on the protein surface. B: The nine unique DMPC lipid molecules (PC1-9) identified per aquaporin-0 monomer. PC1-7 are annular lipids while PC8 and 9 represent bulk lipids (Protein Data Bank accession No. 2B6O).