Exposed loop domains of complexed 14-3-3 proteins contribute to structural diversity and functional specificity

The 14-3-3 family of proteins functions through protein:phosphoprotein interactions, the nature of which has been elucidated using x-ray crystallography. However, some key structural features in nonconserved regions have yet to be fully resolved, leaving open questions regarding the functional selectivity of 14-3-3 family members for diverse clients. In an effort to study surface accessible structural features in 14-3-3 containing macromolecular complexes and to illuminate important structure/function variations among the 14-3-3 isoforms, we determined the epitopes for three unique monoclonal antibodies (mAbs) developed against the Arabidopsis (Arabidopsis thaliana) G-box DNA:protein complex. The epitopes mapped to different loops in a phylogenetically important subset of the 13 14-3-3 family members. All three epitopes were on a common exposed face of complexed 14-3-3s. Two of the mAbs recognized linear sequences within loops 5 and 6, while the third mAb recognized 14-3-3 residues surrounding the pivotal medial Gly in the divalent cation-binding domain of loop 8, together with distal residue(s) in the putative dynamic 10th helix that has yet to be determined by crystallography. Gly at this loop 8 position is unique to nonepsilon 14-3-3 isoforms of the plant kingdom, suggesting that this region constitutes a plant-specific key functional 14-3-3 feature and highlighting that the loop 8 region is functionally significant. Mutagenesis of the medial amino acid in the loop 8 domain changed the flexibility of the C terminus and altered client peptide-binding selectivity, demonstrating the functional significance of the surface accessible, evolutionarily distinct loop 8 domain.