Auxin-transporting ABC transporters are defined by a conserved D/E-P motif regulated by a prolylisomerase

Abstract

The plant hormone auxin must be transported throughout plants in a cell-to-cell manner to affect its various physiological functions. ABCB transporters are critical for this polar auxin distribution, but the regulatory mechanisms controlling their function is not fully understood. The auxin transport activity of ABCB1 was suggested to be regulated by a physical interaction with FKBP42/Twisted Dwarf1 (TWD1), a peptidylprolyl cis-transisomerase (PPIase), but all attempts to demonstrate such a PPIase activity by TWD1 have failed so far.By using a structure-based approach, we identified several surface-exposed proline residues in the nucleotide binding domain and linker of Arabidopsis ABCB1, mutations of which do not alter ABCB1 protein stability or location but do affect its transport activity. P1008 is part of a conserved signature D/E-P motif that seems to be specific for Auxin-TransportingABCBs, which we now refer to as ATAs. Mutation of the acidic residue also abolishes auxin transport activity by ABCB1. All higher plant ABCBs for which auxin transport has been conclusively proven carry this conserved motif, underlining its predictive potential. Introduction of this D/E-P motif into malate importer, ABCB14, increases both its malate and its background auxin transport activity, suggesting that this motif has an impact on transport capacity. The D/E-P1008 motif is also important for ABCB1-TWD1 interactions and activation of ABCB1-mediated auxin transport by TWD1.In summary, our data imply a new function for TWD1 acting as a putative activator of ABCB-mediated auxin transport by cis-trans isomerization of peptidyl-prolyl bonds.

A twist in the ABC: regulation of ABC transporter trafficking and transport by FK506‐binding proteins

“…In light of the clinical importance of CFTR, it is surprising that interacting surfaces and respective prolines have not yet been mapped and/or identified. While for FKBP42/ TWD1-ABCB1 interaction it was shown that the FKBD interferes with the NBD2 containing P1008 [58], FKBP38-CFTR contacts and PPIase-relevant prolines have not yet been determined. Moreover, despite the fact that AtABCB1 P1008 seems to be conserved in the loop between TMD2 and NBD2 of CFTR [58], it will be interesting to see if the three prolines in the R domain of CFTR [86] are directly altering CFTR activity or the affinities for regulatory kinases, or both.…”

“…Strikingly, CFTR opening by cyclophilin A is reversed by cyclosporin A known to inhibit its PPIase activity, providing strong evidence that ABC transport activation can depend on peptidylprolyl isomerization. This concept was recently hardened by demonstrating that transport activation of Arabidopsis ABCB1 by FKBP42/TWD1 was absent, when P1008 located at the surface of NBD2 was mutated to alanine [58]. As said before, direct evidence for any ABC transporter as a substrate for FKBP PPIase is missing and methods to identify PPIase ‘hot’ prolines are not available.…”

“…Also, it is not known which functional domains of FKBP42/TWD1 are involved in both processes, especially if FKBP42/TWD1 owned a hidden PPIase activity similar to FKBP38 (see below). An impact of FKBP42/TWD1 in a module, where it regulates ABCB transport activity by cis-trans isomerization of prolyl bonds, was provided recently by the identification of a surface-exposed, diagnostic D/E-P motif in the NBD2 of ABCBs that do transport auxin [58]. Individual mutation of either E or P in ABCB1 significantly reduced auxin transport; however, the D/ E-P was not essential for interaction with TWD1 [58].…”

“…3). Excitingly, despite belonging to distinct ABC families, P1008 seems to be spatially and structurally conserved in CFTR [58]; P1175 in Fig. 3).…”

Role of ATP‐binding cassette transporters in maintaining plant homeostasis under abiotic and biotic stresses

“…Similarly, ABCB transporters play a very important role in the distribution of auxin inside the plant. The activity of ABCB1 transporter was reported to be regulated by peptidyl prolyl cis ‐ trans isomerase (PPIase; Hao et al, 2020). Further characterization of ABCB1 showed the presence of several proline residues in the domain and linker regions.…”