Defining a Two-Pronged Structural Model for PB1 Domain Interaction in Plant Auxin Responses [Protein Structure and Folding]

April 3rd, 2015 by Korasick, D. A., Chatterjee, S., Tonelli, M., Dashti, H., Lee, S. G., Westfall, C. S., Fulton, D. B., Andreotti, A. H., Amarasinghe, G. K., Strader, L. C., Jez, J. M.

Phox/Bem1p (PB1) domains are universal structural modules that use surfaces of different charge for protein-protein association. In plants, PB1-mediated interactions of AUXIN RESPONSE FACTORs (ARF) and AUXIN/INDOLE 3-ACETIC ACID INDUCIBLE (Aux/IAA) regulate transcriptional events modulated by the phytohormone auxin. Here, we investigate the thermodynamic and structural basis for Arabidopsis thaliana ARF7 PB1 domain self-interaction. ITC and NMR experiments indicate that key residues on both the basic and acidic faces of the PB1 domain contribute to and organize coordinately to stabilize protein-protein interactions. Calorimetric analysis of ARF7PB1 site-directed mutants defines a two-pronged electrostatic interaction. The canonical PB1 interaction between a lysine and a cluster of acidic residues (i.e., OPCA motif) provides one prong with an arginine and a second cluster of acidic residues defining the other prong. Evolutionary conservation of this core recognition feature and other co-varying interface sequences allows for versatile PB1-mediated interactions in auxin signaling.