Peptidyl-prolyl isomerization targets rice Aux/IAAs for proteasomal degradation during auxin signalling

Abstract

In plants, auxin signalling is initiated by the auxin-promoted interaction between the auxin receptor TIR1, an E3 ubiquitin ligase, and the Aux/IAA transcriptional repressors, which are subsequently degraded by the proteasome. Gain-of-function mutations in the highly conserved domain II of Aux/IAAs abolish the TIR1-Aux/IAA interaction and thus cause an auxin-resistant phenotype. Here we show that peptidyl-prolyl isomerization of rice OsIAA11 catalysed by LATERAL ROOTLESS2 (LRT2), a cyclophilin-type peptidyl-prolyl cis/trans isomerase, directly regulates the stability of OsIAA11. NMR spectroscopy reveals that LRT2 efficiently catalyses the cis/trans isomerization of OsIAA11. The lrt2 mutation reduces OsTIR1-OsIAA11 interaction and consequently causes the accumulation of a higher level of OsIAA11 protein. Moreover, knockdown of the OsIAA11 expression partially rescues the lrt2 mutant phenotype in lateral root development. Together, these results illustrate cyclophilincatalysed peptidyl-prolyl isomerization promotes Aux/IAA degradation, as a mechanism regulating auxin signalling.

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

“…The unusual FK506-binding domain in TWD1 was instead found to participate in protein interaction with ABCB-type auxin transporters and is apparently involved in the functional regulation of these transporters [44] (see Section 4). TWD1 also possesses a tetratricopeptide repeat domain (common [131] Regulation and PM-to-ER trafficking of ABCB-type auxin transporters [8,9] Z. mais [133] Interaction with ABCB transporters on PM (via FKB-domain) [8,62], ABCC transporters on tonoplast (via TPR domain) [66], with HSP90 (TPR domain) [62,65] and calmodulin (calmodulin-binding domain) [66] Over-expression of TWD1 lacking its membrane anchor results in hypermorphic growth [64] FKBP72/PAS1 A. thaliana [69] Unclear (C-terminal membrane anchor), nuclear [68,134] Low, inhibited by FK506 and rapamycin [134] Up-regulated cell division, leaf fusions, short hypocotyls, sterile [69] O. sativa [131] Chaperon during translocation of NAC-like transcription factor (AtFAN) into nucleus [68] Z. mais [133] Regulation of very long fatty acid elongation [70] -DGT/CypA/ROC1/CYP1/CYP2/ P. patens [10] Nuclear and cytoplasmic [10,90], phloem sieve elements [79] Significant [77,78], inhibited by CsA [78] Regulates growth [90], gene expression [84,90,92], patterned cell division [88], phloem function [79], ROS balance in root apical meristem [87] A. [137] in many large immunophilins) that was shown to interact with vacuolar ABC transporters of the C subclass and HSP90 [65,66], as w...…”

“…2), and can move from shoot to root in grafted plants partially restoring the lack of lateral roots in dgt mutant rootstocks [89]. A DGT-like small cyclophilin from Physcomitrella patens is also important for auxin-regulated growth [90], whereas a related cyclophilin from rice (LATERAL ROOTLESS2, LRT2/OsCYP2) plays a crucial role in auxin-regulated lateral root formation [91] and has been shown to directly regulate the stability of the auxin-responsive transcriptional repressor protein, OsIAA11, via peptidylprolyl isomerization [92]. The Arabidopsis cyclophilin CYCLOPHILIN20-2 regulates flowering time by modulating the conformation of BRASSINAZOLE-RESISTANT1 in Ref.…”

Mechanisms of auxin signaling

“…For example, S-nitrosylation of conserved cysteine residues in the LRR of TIR1 can enhance the interaction between TIR1 and Aux/IAA proteins (Terrile et al, 2012). In addition, the cyclophilin LATERAL ROOTLESS2 (LRT2) was recently shown to facilitate TIR1/AFB-Aux/IAA interaction in rice by allowing for correct folding of the degron motif-containing domain (Jing et al, 2015).…”

1H, 13C and 15N NMR assignments of cyclophilin LRT2 (OsCYP2) from rice

“…Notably, auxin is required for the E3 ubiquitin ligase SCF TIR1/AFB complex to bind specifically with the cis isomer of a highly conserved Trp-Pro degron motif in Aux/IAA proteins such as OsIAA11, thereby targeting these transcription repressor proteins for proteasomal degradation. In vitro interaction between LRT2 and OsIAA11 and LRT2 catalysis of the Trp-Pro peptide bond in the Aux/IAA degron motif of OsIAA11 has been reported (Jing et al 2015). This interaction is of importance in the field of plant biology, since LRT2 catalysis of cis/trans isomerization of OsIAA11 could accelerate ubiquitin-mediated proteasomal degradation of OsIAA11 via the cis conformation of the conserved Aux/IAA degron motif (Jing et al 2015).…”