The signalling molecule auxin provides instructive cues guiding the entire plant ontogenesis. Its orchestrating role largely depends on the directional transport between cells facilitated by a plant-specific group of auxin transporters named PINs. While the physiological and developmental roles of PINs along with their intracellular dynamics have been extensively studied, the biochemical and structural underpinnings of PIN activity are still scarcely characterized. PINs are plasma membrane (PM) localized auxin transporters. The so called "long" PINs have a ten transmembrane topology with a large loop separating the two bundles of five alpha helices. In this project we want to analyse the importance of certain motifs in the sequence of the PM PINs. In particular, we want to asses the functional significance of the C-terminus and consequently the second alpha-helical bundle (helices 6 -10) of PM PINs.