title: Identification of a dimeric intermediate in the unfolding pathway for the calcium-binding protein S100B.
abstract: The S100 proteins comprise 25 calcium-signalling members of the EF-hand protein family. Unlike typical EF-hand signalling proteins such as calmodulin and troponin-C, the S100 proteins are dimeric, forming both homo- and heterodimers in vivo. One member of this family, S100B, is a homodimeric protein shown to control the assembly of several cytoskeletal proteins and regulate phosphorylation events in a calcium-sensitive manner. Calcium binding to S100B causes a conformational change involving movement of helix III in the second calcium-binding site (EF2) that exposes a hydrophobic surface enabling interactions with other proteins such as tubulin and Ndr kinase. In several S100 proteins, calcium binding also stabilizes dimerization compared to the calcium-free states. In this work, we have examined the guanidine hydrochloride (GuHCl)-induced unfolding of dimeric calcium-free S100B. A series of tryptophan substitutions near the dimer interface and the EF2 calcium-binding site were studied by fluorescence spectroscopy and showed biphasic unfolding curves. The presence of a plateau near 1.5 M GuHCl showed the presence of an intermediate that had a greater exposed hydrophobic surface area compared to the native dimer based on increased 4,4-dianilino-1,1'-binaphthyl-5,5'-disulfonic acid fluorescence. Furthermore, (1)H-(15)N heteronuclear single quantum coherence analyses as a function of GuHCl showed significant chemical shift changes in regions near the EF1 calcium-binding loop and between the linker and C-terminus of helix IV. Together these observations show that calcium-free S100B unfolds via a dimeric intermediate.
authors.affiliation: Department of Biochemistry, The University of Western Ontario, London, Ontario, Canada N6A 5C1. gshaw1@uwo.ca
authors.forename: Gary S
authors.initials: GS
authors.lastname: Shaw
authors.forename: Nicole M
authors.initials: NM
authors.lastname: Marlatt
authors.forename: Peter L
authors.initials: PL
authors.lastname: Ferguson
authors.forename: Kathryn R
authors.initials: KR
authors.lastname: Barber
authors.forename: Stephen P
authors.initials: SP
authors.lastname: Bottomley
mesh_terms: D002942:Circular Dichroism; D019281:Dimerization; D019791:Guanidine / Q000737:chemistry; D006801:Humans; D008958:Models, Molecular; D008970:Molecular Weight; D009414:Nerve Growth Factors / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D011489:Protein Denaturation; D017510:Protein Folding; D020836:Protein Structure, Quaternary; D064568:S100 Calcium Binding Protein beta Subunit; D009418:S100 Proteins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D014364:Tryptophan / Q000737:chemistry
chemical_list: D009414:Nerve Growth Factors; D064568:S100 Calcium Binding Protein beta Subunit; D009418:S100 Proteins; C579160:S100B protein, human; D014364:Tryptophan; D019791:Guanidine