Weak Specific Interactions of Proteins Assessed by NMR
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Weak interactions of proteins are often ascribed to non-specific contacts of surface residues. In particular calorimetric methods of fluorescence based assays are considered meaningful only if a threshold of micromolar dissociation constants is surpassed. We have been studying a broad range of protein interactions, where NMR methods could clearly show that interactions considered too weak for being ?specific? show the highly selective involvement of regions, which in the static structures are characterized as intrinsically disordered. First, this was observed by group selective saturation transfer difference experiments (gs-STD)  for the bacterial protein acpP  interacting with a synthetic inhibitory peptide. Second, the assembly of the oxygen evolving complex in higher plants involves three proteins (PsbP, PsbQ and PsbO) [3,4] and depends on interactions of their disordered regions, as was assessed by CEST experiments. Finally, in the human protein STIM1, which is involved in Calcium-channel activation, weak interactions, which are hardly affecting the chemical shifts but cause highly residue-specific changes in relaxation parameters, cooperatively trigger conformational transitions and oligomer formation. In spite of the vastly different sources and functions of these proteins, a general picture emerges: Cooperative protein interactions often involve weak specific contacts, which occur in a conformational state of low population and thus, individually, cause only minor perturbations in chemical shifts. Site specific NMR relaxation parameters are sensitive to these weak interactions and can be used to discriminate between selective and non-selective interactions.