Master and servant: Regulation of auxin transporters by FKBPs and cyclophilins

Abstract

Plant development and architecture are greatly influenced by the polar distribution of the essential hormone auxin. The directional influx and efflux of auxin from plant cells depends primarily on AUX1/LAX, PIN, and ABCB/PGP/MDR families of auxin transport proteins. The functional analysis of these proteins has progressed rapidly within the last decade thanks to the establishment of heterologous auxin transport systems. Heterologous co-expression allowed also for the testing of protein-protein interactions involved in the regulation of transporters and identified relationships with members of the FK506-Binding Protein (FKBP) and cyclophilin protein families, which are best known in non-plant systems as cellular receptors for the immunosuppressant drugs, FK506 and cyclosporin A, respectively. Current evidence that such interactions affect membrane trafficking, and potentially the activity of auxin transporters is reviewed. We also propose that FKBPs andcyclophilins might integrate the action of auxin transport inhibitors, such as NPA, on members of the ABCB and PIN family, respectively. Finally, we outline open questions that might be useful for further elucidation of the role of immunophilins as regulators (servants) of auxin transporters (masters).

Ectopic expression of a pea apyrase enhances root system architecture and drought survival in Arabidopsis and soybean

“…The effects of auxin on root growth are complex, both promotive and inhibitory (Street et al, 2016). However, auxin transport promotes both lateral root initiation (Casimiro et al, 2001) and root hair initiation and differentiation (Jones et al, 2008), and both auxin distribution and ABCB transporters strongly impact RSA (Geisler, Bailly, & Ivanchenko, 2016). Key questions to be resolved would be whether eATP and apyrases regulate auxin transport in soybean and other plants as it does in Arabidopsis and whether FIGURE 10 Speculative model proposing pathways by which the heightened expression of psNTP9 in soybeans could lead to an expanded RSA and an increase in seed yield More drought tolerant and more ABA responsive Distéfano et al (2015) psNTP9 exerts its effects on RSA primarily by its function on the plasma membrane to limit [eATP] or by its function in the nucleus (Chen et al, 1987), where altering NTP levels would have obvious effects on transcription.…”

TWISTED DWARF1 Mediates the Action of Auxin Transport Inhibitors on Actin Cytoskeleton Dynamics

“…The plasma membrane presence of ABCBs is dependent on the FKBP42 (FK506 binding protein) TWISTED DWARF1 (TWD1) acting as a chaperone of endoplasmic reticulum (ER) to PM provision of ABCB1, ABCB4, and ABCB19 (Wu et al, 2010;Wang et al, 2013;Zhu and Geisler, 2015;Geisler et al, 2016). Therefore, these ABCBs, but not PIN1 or PIN2, are delocalized and degraded in twd1 (Bouchard et al, 2006;Wu et al, 2010;Wang et al, 2013;Bailly et al, 2014).…”

A conserved D/E-P motif in the nucleotide binding domain of plant ABCB/PGP-type ABC transporters defines their auxin transport capacity

“…In Arabidopsis, so-called long PINs (PIN1, 2, 3, 4 and 7) function as plasma membrane (PM) permeases (5), while cytoplasmic entry over the plasma membrane was shown to be dependent on AUX1/LAX proteins thought to function as highaffinity auxin-proton symporters (7,8). In contrast, a subgroup of auxin-transporting ABCBs (referred to as ATAs in the following) functions as primary active (ATP-dependent) auxin pumps that are able to transport against steep auxin gradients (9)(10)(11). Out of the 22 full-size ABCB isoforms in Arabidopsis, ABCB1, 4, 6, 14, 15, 19, 20 and 21, were associated with polar auxin transport (PAT) (12)(13)(14)(15)(16)(17), however, only for ABCB1, 4,6,19,20,21, transport activities were confirmed (11)(12)(13)15,16,(18)(19)(20).…”

“…1). All characterized auxin transporters have been shown to be regulated on the transcriptional and non-transcriptional level (9,24,25). PM PINs and ABCBs were shown to be regulated by a partially overlapping subset of members of the AGC kinase family (26,27).…”

“…While protein phosphorylation of the central PIN loop seems to alter both their polarity and transport activity (28), phosphorylation of the so-called ABCB linker has been described to result only in activity changes (26)(27)(28). Further, PINs and ABCBs seem to be also regulated by protein-protein interaction with cis-trans peptidylprolyl isomerases (PPIases), which are formed by three evolutionary unrelated families: the cyclophilins, the FK506-Binding Proteins (FKBPs) and the parvulins (9). The cyclophilin, DIAGEOTROPICA (DGT/ CypA), seems to decrease the functionality of PINs and their presence on the PM (29) although underlying mechanisms are far from being understood.…”