Plant Major Intrinsic Proteins - natural variation and evolution

Abstract: Major Intrinsic Proteins (MIPs, also called Aquaporins, AQPs) are channel forming membrane proteins. Although initially functionally characterized and named after their water channeling property in human red blood cells, it has become increasingly evident that MIPs are present in all types of organisms and transport a variety of small, uncharged molecules besides water. MIPs have a highly conserved structure with a constriction region and electrostatic repulsion filter allowing the combination of high transport rate and selectivity, characteristic of MIPs. In plants, MIPs form a large and varied protein family, with roughly three times as many isoforms as found in animals. Even though the abundance implies that MIPs have important functions in plants, the roles of individual MIPs have so far only been described for a handful of isoforms. Variations in the filter regions as well as experimental data, suggest differences in substrate specificities and localization for different MIP subfamilies. However actual functions of different isoforms remain largely unknown as traditional knock out/knock down experiments to a large extent have failed to reveal any clear phenotypes. Using another approach, we used naturally occurring genetic variants (accessions) of the model plant Arabidopsis thaliana to see if differences in traits, such as drought tolerance, could be linked to differences in MIP regulation. We also looked at the evolution of the MIP family in plants, to see if this could for example be linked to events such as the emergence of a vascular system or the development of a terrestrial lifestyle. Therefore we identified the whole MIP family in the moss Physcomitrella patens and in nine different green algae and investigated how these relate to those of higher plants. An amazing diversity of MIPs was found and surprisingly some of the higher plant subfamilies were present also in these simple plants.