The Aquaporins - Expression and Regulation of Water Channel Proteins

Abstract: Aquaporins are integral membrane proteins that function as water channels highly specific for water or as aquaglyceroporins permeable to water as well as small solutes such as glycerol and urea. The model plant Arabidopsis thaliana has 35 genes encoding aquaporin proteins. Identifying the complete set of aquaporins in Arabidopsis constitutes an unique opportunity to comprehensively classify and rename all aquaporin family members in Arabidopsis, creating a more uniform nomenclature. It is not understood why the plant needs so many genes, but they are most likely differentially expressed at distinct developmental stages and at stress situations. Regulation of aquaporin expression gives the plant the means to change the water permeability of different membranes. The knowledge of the aquaporin expression patterns is required to understand the need for the large number of proteins and their specific function. The expression pattern was studied in detail for the spinach vacuolar aquaporin SoTIP2;1 using immunocytochemical techniques. Another way of modulating the membrane permeability is by regulation of the activity of the aquaporins, for instance by phosphorylation. This is the case for the spinach plasma membrane aquaporin PM28A. We have shown that the water transport activity of PM28A increases upon phosphorylation at two sites. This provides the plant with a possibility to open and close the water channel and thereby prevent water loss during drought stress. In order to be able to study the molecular structure of PM28A, we decided to express the protein heterologously. The methylotrophic yeast Pichia pastoris was proven to be a good choice for overexpression, since we could obtain extremely high amounts of pure and functional PM28A.