Conformational dynamics underlie the activity of the auxin-binding protein, Nt-abp1

The auxin-binding protein 1 (ABP1) has been proposed to be involved in the perception of the phytohormone, at the plasma membrane. Site-directed mutag enesis was performed on highly conserved residues at the C terminus of ABP1 to investigate their relative importance in protein folding and activation of a functional response at the plasma membrane. Detailed analysis of the dynamic interaction of the wild-type ABP1 and mutated proteins with three d istinct monoclonal antibodies recognizing conformation-dependent epitopes w as performed by surface plasmon resonance. The influence of auxin on these interactions was also investigated. The Cys(177) as well as Asp(175) and Gl u(176) were identified as critical residues for ABP1 folding and action at the plasma membrane. On the contrary, the C-terminal KDEL sequence was demo nstrated not to be essential for auxin binding, interaction with the plasma membrane, or activation of the transduction cascade although it does appea r to be involved in the stability of ABP1. Taken together, the results conf irmed that ABP1 conformational change is the critical step for initiating t he signal from the plasma membrane.