Book of Abstracts: Albany 2007
June 19-23 2007
Aquaporins facilitate the flow of water across biological membranes at almost the diffusion rate. These integral membrane proteins are highly selective for water (in some cases also other small polar molecules) yet they do not allow the movement of protons. From the first structures of members of the aquaporin family [1,2] and molecular dynamics simulations a consensus has emerged concerning the mechanism of selective water transport and proton extrusion . As such emphasis has shifted towards understanding the mechanisms of aquaporin regulation. In eukaryotes aquaporins are frequently gated, either by being directed into various membranes (trafficking) or by regulation of their water-transport activity in the membrane (gating).
Plants counteract fluctuations in water supply by regulating all aquaporins in the cell plasma membrane. Channel closure results either from the dephosphorylation of two conserved serine residues under conditions of drought stress, or from the protonation of a conserved histidine residue following a drop in cytoplasmic pH due to anoxia during flooding (Fig. 1). In this lecture I will present X-ray structures  of the spinach plasma membrane aquaporin SoPIP2;1 in both its closed (2.1 Å resolution) and open (3-9 Å resolution) conformations (Fig. 2).
1Department of Chemistry, Biochemistry & Biophysics, Göteborg University, 40530 Göteborg, Sweden.
Figure 1: Schematic illustrating the biochemical mechanism whereby the plant plasma membrane aquaporins are gated. Under conditions of drought stress these aquaporins are closed by the dephospohrylation of highly conserved serine residues. Under conditions of flooding a lack of oxygen causes the cytosolic pH to decrease, which in turn protonates a conserved histidine and thereby closes the channel.
Figure 2: X-ray structures of the plant aquaporin SoPIP2;1 in its closed (2.1 Å resolution) and open (3.9 Å resolution). The channel profile maps (dotted lines and blue/green line-outs on the left) illustrate how the closed structure is opened by a movement of loop D.
Results from molecular dynamics simulations will also be presented to help illustrate the initial conformational changes triggering gating. These structural studies revealed that loop D plays a key role capping the channel from the cytoplasm and thereby occluding the pore in the closed conformation. In the open conformation loop D is displaced up to 16Å and this movement opens the channel entrance from the cytoplasm. This molecular gating mechanism appears to be highly conserved throughout all plant plasma membrane aquaporins. Finally, the extent to which this structural mechanism sheds light on the regulation of other aquaporins in other organisms will be discussed .
References and Footnotes