My group has a long-lasting interest and competence in the characterization of novel plant hormone transporters.
However, our focus lies in exploring the individual roles of transport proteins driving the fascinating polar, cell-to-cell movement of the plant signaling compound, auxin. This process, called polar auxin transport (PAT), represents a unique, plant-specific mechanism that has puzzled mankind since its first description by the Darwins.
Over the years we have been focusing on the characterization of ABCB-type auxin transporters, belonging to the superfamily of ABC transporters. In contrast to PIN-proteins, ABCB function as ATP-dependent, auxin pumps enabling them to transport auxin against steep gradient. Interestingly, unlike mammalian ABCB proteins - catalyzing the export of a plethora of structurally unrelated substrates, such as anti-cancer drugs - their plant orthologs were shown to own a narrow specificity towards structurally related auxinic compounds. Another striking difference is that some plant ABCBs have been characterized as facultative im/exporters.
Using a combination of biochemical, in silico and imaging techniques, we are currently deciphering their individual modes of regulation, their functional interaction with other transporters and regulatory components and their impact on auxin-mediated plant physiology with an emphasis on root gravitropism.
Our special interest lies in the physical and functional interaction between the auxin exporter, ABCB1, and its regulatory interacting protein, the FKBP42, TWISTED DWARF1 (TWD1). TWD1 serves as a chaperone during ER to PM secretion of auxin-transporting ABCBs: in analogy to regulation of Cystic Fibrosis Conductance Regulator (CFTR/ABCC7) by human FKBP38, ABCBs are degraded in the absence of TWD1/FKBP42.
We identified the AGCIII kinase, PINOID, as a valid partner in interaction with TWD1. Phosphor-proteomics analyses uncovered S634 as a key residue of the regulatory ABCB1 linker and a very likely target of PINOID phosphorylation that determines both transporter transport activity and drug binding.
Recently, we have shown that TWD1is involved in the regulation of actin filament organization and dynamics. Moreover, TWD1 is required for actin cytoskeleton remodeling by the auxin transport inhibitor, NPA (1-N-naphthylphthalamic acid). The TWD1-ACTIN7 module controls plasma membrane presence of efflux transporters and as a consequence act7 and twd1 share developmental and physiological phenotypes indicative of defects in auxin transport. This function appears to be evolutionary conserved since TWD1 expression in budding yeast alters actin polarization and cell polarity and provides NPA sensitivity.
In an ESA-funded project in collaboration with Dr. Marcel Egli (HSLU) we are currently imaging auxin gradients and expression of auxin transporters under microgravity conditions using a so-called Random Positioning Machine. These analyses will allow us to get new insights how plants respond to changed environmental stimuli, such as gravity.
We have provided evidence that the ABC transporter ABCG36/PDR8/PEN3 catalyses the export of the auxin precursor, IBA. Recent evidence, however, suggest that ABCG36 might also export the phytoalexin, camalexin, which would place this transporter at the fascinating crossroad between plant growth and defence.
Our work indicates that this decisision - either to grow or to fight - on a molecular level is controlled by protein phosphorylation of ABCG36 privuded by the recepror kinase, QSK1.