Solution NMR Structure of S100B Bound to the High-affinity Target Peptide TRTK-12

The solution NMR structure is reported for Ca2+-loaded S100B bound to a 12-residue peptide, TRTK-12, from the actin capping protein CapZ (α1 or α2 subunit, residues 265–276: TRTKIDWNKILS). This peptide was discovered by Dimlich and co-workers by screening a bacteriophage random peptide display library, and it matches exactly the consensus S100B binding sequence ((K/R)(L/I)XWXXIL). As with other S100B target proteins, a calcium-dependent conformational change in S100B is required for TRTK-12 binding. The TRTK-12 peptide is an amphipathic helix (residues W7 to S12) in the S100B–TRTK complex, and helix 4 of S100B is extended by three or four residues upon peptide binding. However, helical TRTK-12 in the S100B–peptide complex is uniquely oriented when compared to the three-dimensional structures of other S100–peptide complexes. The three-dimensional structure of the S100B–TRTK peptide complex illustrates that residues in the S100B binding consensus sequence (K4, I5, W7, I10, L11) are all involved in the S100B–peptide interface, which can explain its orientation in the S100B binding pocket and its relatively high binding affinity. A comparison of the S100B–TRTK peptide structure to the structures of apo- and Ca2+-bound S100B illustrates that the binding site of TRTK-12 is buried in apo-S100B, but is exposed in Ca2+-bound S100B as necessary to bind the TRTK-12 peptide.