title: Analysis of the structure of human apo-S100B at low temperature indicates a unimodal conformational distribution is adopted by calcium-free S100 proteins.
abstract: S100B is one of the best-characterized members of the calcium-signaling S100 protein family. Most S100 proteins are dimeric, with each monomer containing two EF-hand calcium-binding sites (EF1, EF2). S100B and other S100 proteins respond to calcium increases in the cell by coordinating calcium and undergoing a conformational change that allows them to interact with a variety of cellular targets. Although several three dimensional structures of S100 proteins are available in the calcium-free (apo-) state it has been observed that these structures appear to adopt a wide range of conformations in the EF2 site with respect to the positioning of helix III, the helix that undergoes the most dramatic calcium-induced conformational change. In this work, we have determined the structure of human apo-S100B at 10 degrees C to examine whether temperature might be responsible for these structural differences. Further, we have used this data, and other available apo-S100 structures, to show that despite the range of interhelical angles adopted in the apo-S100 structures, normal Gaussian distributions about the mean angles found in the structure of human apo-S100B are observed. This finding, only obvious from the analysis of all available apo-S100 proteins, provides direct structural evidence that helix III is a loosely packed helix. This is likely a necessary functional property of the S100 proteins that facilitates the calcium-induced conformational change of helix III. In contrast, the calcium-bound structures of the S100 proteins show significantly smaller variability in the interhelical angles. This shows that calcium binding to the S100 proteins causes not only a conformational change but results in a tighter distribution of helices within the EF2 calcium binding site required for target protein interactions.
authors.affiliation: Department of Biochemistry, The University of Western Ontario, London, Ontario N6A5C1, Canada.
authors.forename: Shahid
authors.initials: S
authors.lastname: Malik
authors.forename: Matthew
authors.initials: M
authors.lastname: Revington
authors.forename: Steven P
authors.initials: SP
authors.lastname: Smith
authors.forename: Gary S
authors.initials: GS
authors.lastname: Shaw
mesh_terms: D001059:Apoproteins / Q000737:chemistry*; D002118:Calcium / Q000737:chemistry*; D006801:Humans; D009414:Nerve Growth Factors / Q000737:chemistry*; D011487:Protein Conformation; D011994:Recombinant Proteins / Q000737:chemistry; D064568:S100 Calcium Binding Protein beta Subunit; D009418:S100 Proteins / Q000737:chemistry*; D013696:Temperature
chemical_list: D001059:Apoproteins; D009414:Nerve Growth Factors; D011994:Recombinant Proteins; D064568:S100 Calcium Binding Protein beta Subunit; D009418:S100 Proteins; C579160:S100B protein, human; D002118:Calcium