Transport properties of canonical PIN-FORMED proteins and the role of the loop domain in auxin transport

Indole-3-acetic acid (IAA), the most abundant endogenous auxin is transported in plants in a polar fashion by PIN-FORMED (PIN) transporters and controls virtually all plant growth and developmental processes. Canonical PINs possess a long and largely disordered cytosolic loop domain which is shorter in non-canonical PINs. Auxin transport by canonical PINs is activated loop phosphorylation by kinases. While the structure of the transmembrane domains of these transporters was recently solved, their transport properties remained poorly characterized and particularly the relative roles of the transmembrane and loop domain therein. In this study we used flux studies to obtain quantitative kinetic parameters of IAA transport mediated by canonical PINs as well as of chimeras between transmembrane and loop domains of different PINs upon their activation by D6 PROTEIN KINASE or PINOID. We found that the transporters possess distinct transport properties that are due to both the transmembrane and loop domain. To demonstrate the physiological relevance of these distinct transport properties, we modelled root tip IAA distribution patterns and investigated the potential of different PINs to complement the agravitropic root growth phenotype of the pin2 mutant when expressed in the PIN2 domain. We found a strong correlation between transport parameters and physiological output indicating that in addition to PIN polarity a low transport rate in the PIN2 expression domain is required for gravitropic growth. Overall, the data show that the loop domain is not only required for activation of PIN-mediated auxin transport but has an additional role in the transport cycle by a currently unknown mechanism.