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Lucia Strader

Washington University in St. Louis
Structural insights into plant auxin signal transduction
Auxin signaling guides plants through nearly every aspect of growth and development. PB1 domain interactions among AUXIN RESPONSE FACTOR (ARF) and AUXIN/INDOLE 3-ACETIC ACID (Aux/IAA) proteins regulate auxin-responsive gene transcription. Understanding the driving forces behind these protein-protein interactions that facilitate auxin signal transduction is vital. The crystal structure of the C-terminal interaction domain of Arabidopsis ARF7 reveals a Phox and Bem1p (PB1) domain that provides both positive and negative electrostatic interfaces for directional protein interaction. In this work, we investigate the attractive forces that influence ARF-ARF self-interaction. We found using isothermal titration calorimetry and NMR chemical shift experiments that key residues on both the basic and acidic faces of the domain contribute to and organize coordinately to stabilize PB1 domain binding. Our data suggest the positively charged residues on the basic face of the PB1 domain drive the interaction, whereas the less structured acidic domain face coordinates around these residues to anchor the binding. These thermodynamic and structural analyses uncover the driving forces and key residues involved in PB1 domain interactions that universally organize cellular signaling scaffolds. Our data suggest a revised model of auxin response in which ARF and Aux/IAA multimerization are necessary for regulation of auxin response.
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