JA-Ile, the endogenous bioactive JA, is perceived by the F-box protein CORONATINE INSENSITIVE1 (COI1) that functions as the hormone receptor. In the presence of JA-Ile, the E3 ubiquitin ligase complex SCF COI1 recognizes its targets, the JAZ proteins that are subsequently ubiquitinylated and destroyed by the 26S proteasome. [1][2][3][4][5] In an unelicited state, the JAZ proteins bind and inactivate the transcription factor MYC2, thereby repressing the activation of early JA-responsive genes through a mechanism that remained elusive to date.In our study, 6 we used a tandem affinity purification (TAP) technology platform that had been established in Arabidopsis Dissection of the one-MegaDalton JAZ1 protein complex Jan Geerinck, Laurens Pauwels, Geert De Jaeger and Alain Goossens* Department of Plant Systems Biology; VIB and Department of Plant Biotechnology and Genetics; Ghent University; Ghent, Belgium thaliana cell suspension cultures, 7 to retrieve new interactors of JAZ1 and, thereby, unravel the core JA signaling module. JAZ1-TAP cultures were mock treated or elicited with JA for 1 min and subsequently harvested and analyzed. JAZ1 was found to interact with JAZ12 and MYC3, a close relative of MYC2. Also interaction between COI1 and JAZ1 was observed, but only in the presence of JA, in accordance with the proposed models.1-5 We focused on a previously uncharacterized protein, designated NINJA (At4g28910) that was retrieved with JAZ1 independently of the JA elicitation. Studies with green fluorescent protein (GFP)-tagged proteins demonstrated that the stability of NINJA was not affected by JAs, in contrast to the JAZ proteins that were degraded within minutes after JA application.The specificity of NINJA for JAZ proteins was confirmed by yeast-two hybrid (Y2H) analysis and pull-down experiments. Furthermore, these experiments revealed that NINJA interacted with most JAZ proteins as well as with other ZIMdomain proteins that contain the conserved TIF[F/Y]XG (TIFY) motif and belong to the group II TIFY proteins, 8 such as PEAPOD1 (PPD1), PPD2 and TIFY8. In a Y2H analysis with deletion series of JAZ1, a 39-amino-acid fragment with the TIFY motif was found to be necessary and sufficient for the interaction with NINJA. Conversely, a deletion series of NINJA that is characterized by three conserved protein motifs, designated A, B and C, showed that the C-domain was responsible and sufficient for interaction with JAZ proteins.Analogously to the JAZ1-TAP, a TAP analysis with NINJA as bait revealed that NINJA was present in a complex with the