Calcium-dependent and -independent interactions of the S100 protein family

Abstract

The S100 proteins comprise at least 25 members, forming the largest group of EF-hand signalling proteins in humans. Although the proteins are expressed in many tissues, each S100 protein has generally been shown to have a preference for expression in one particular tissue or cell type. Three-dimensional structures of several S100 family members have shown that the proteins assume a dimeric structure consisting of two EF-hand motifs per monomer. Calcium binding to these S100 proteins, with the exception of S100A10, results in an approx. 40 degrees alteration in the position of helix III, exposing a broad hydrophobic surface that enables the S100 proteins to interact with a variety of target proteins. More than 90 potential target proteins have been documented for the S100 proteins, including the cytoskeletal proteins tubulin, glial fibrillary acidic protein and F-actin, which have been identified mostly from in vitro experiments. In the last 5 years, efforts have concentrated on quantifying the protein interactions of the S100 proteins, identifying in vivo protein partners and understanding the molecular specificity for target protein interactions. Furthermore, the S100 proteins are the only EF-hand proteins that are known to form both homo- and hetero-dimers, and efforts are underway to determine the stabilities of these complexes and structural rationales for their formation and potential differences in their biological roles. This review highlights both the calcium-dependent and -independent interactions of the S100 proteins, with a focus on the structures of the complexes, differences and similarities in the strengths of the interactions, and preferences for homo- compared with hetero-dimeric S100 protein assembly.

Figure 1. Calcium-dependent and -independent interactions of the S100 family

(A) S100 proteins generate diverse physiological responses by interacting with target molecules (pink and yellow). At low calcium concentrations, S100 proteins (orange) reside in their calcium-free (apo) state. Upon influx of calcium via voltage-gated or receptor-mediated channels, the S100 protein binds calcium and undergoes a conformational change that modifies its hydrophobic surface properties. This allows the protein to interact with a wide spectrum of target proteins (yellow) to stimulate a physiological response. Release of calcium through Ca²⁺-ATPase activity results in the dissociation of calcium and target protein from the S100 protein, returning it to its apo state. Although the majority of S100-target interactions are calcium-dependent, some S100 members have been shown to interact with target proteins (pink) in a calcium-independent fashion. (B) Dimeric S100 proteins can exchange subunits with other members of the S100 proteins to form homo- and hetero-dimers in the same cell type as shown by in vitro and in vivo experiments. The populations of each species are dependent on the concentration of the S100 protein in the cell and the relative affinities for the S100 homo- and hetero-dimers.

Figure 2. Calcium-dependent conformational change in S100 proteins

The three-dimensional structures of calcium-free S100A11 (apo-S100A11) and calcium-bound S100A11 (Ca-S100A11) are shown to demonstrate the calcium-induced conformational change. In the symmetrical dimer, helices of one monomer (I–IV) are highlighted in different colours, while the other monomer (helices I'–IV') is coloured grey. As sensors, the S100 proteins experience a conformational change upon calcium binding (four atoms/dimer). The rearrangement of helix III (orange) exposes previously buried residues that are essential for target recognition (not shown, see Figure 3) and further biological response. This Figure was drawn using MacPyMOL (http://delsci.com/macpymol/).

Figure 3. Target protein orientation for S100 proteins

A ribbon representation of calcium-bound S100A11 is presented, showing one of the monomers in light grey (helices labelled as I, II, III and IV) and the other monomer in dark grey (helices labelled as I' and IV'). The S100 portion from the three dimensional complexes of S100A10–annexin A2, S100B–p53, S100B–TRTK and S100B–NDR kinase was superimposed with S100A11 to give the relative orientations of each of the target peptides. Annexin A1 (residues 1–11) is shown in pink, annexin A2 (residues 1–11) in yellow, NDR kinase (residues 71–87) in cyan, p53 (residues 99–112) in purple and TRTK-12 (residues 1–12) in both green and blue. The N- and C-termini of each of the peptides are indicated with either an N or C respectively. The S100A11 and S100B complexes are in the calcium-bound state, although no calcium ions are shown in the Figure. S100A10 is in the calcium-free state. This Figure was drawn using MacPyMOL (http://delsci.com/macpymol/).