Of the main plant hormones, our understanding of auxin signalling is better than most. Auxin is synthesized in newly formed organs and is then transported to the rest of the plant. In Arabidopsis, the directionality of this transport is thought to result from the static, polar distribution of putative auxin-efflux carriers called PINs. Reporting in Nature, Geldner et al. show that one of these, PIN1, does not stay at the plasma membrane, but that it rapidly cycles between the plasma membrane and an unknown cellular compartment. Their results further indicate that auxin-specific transport inhibitors can actually disturb general transport of membrane proteins.

Previous studies have shown that PIN1 is found at the basal end of cells (figure, top), and that this polar localization is required for normal transport of auxin. gnom mutants are defective in normal PIN1 localization, and they fail to establish tissue polarity (and presumably normal auxin transport). Geldner et al. investigated why PIN1 is mislocalized in gnom mutants.

GNOM encodes a brefeldin A (BFA)-sensitive regulator of vesicular transport. Treating wild-type roots with BFA abolished membrane localization of PIN1, which leads to its accumulation in an intracellular compartment (figure, bottom). This process is rapidly reversible, insensitive to protein-synthesis inhibitors, and dependent on actin. These remarkable results indicate that the highly stable pool of PIN1 protein rapidly cycles between the plasma membrane and an undefined intracellular compartment along the cytoskeleton.

How, then, do the 'auxin-transport' inhibitors work? Geldner et al. examined the effects of an inhibitor called TIBA in concert with BFA. Pre- or co-treatment with TIBA prevented PIN1 from leaving the plasma membrane, and treating roots with TIBA after BFA prevented PIN1 from returning to the plasma membrane after BFA was removed. The effect of TIBA is not specific to PIN1, so these results indicate that TIBA could interfere with the general movement of proteins in the cell, at the level of the transport route itself.