Mouse S100G Protein Exhibits Properties Characteristic of a Calcium Sensor

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CDcircular dichroism spectroscopy, circular dichroism spectroscopy, DSCdifferential scanning calorimetry, differential scanning calorimetry, EDTAethylenediaminetetraacetic acid, ethylenediaminetetraacetic acid, EGTAethylene glycolbis(2-aminoethylether)-N, N, N’, N’-tetraacetic acid, ethylene glycolbis(2-aminoethylether)-N, N, N’, N’-tetraacetic acid, ESI-MSelectrospray ionization mass spectrometry, electrospray ionization mass spectrometry, GdnHClguanidine hydrochloride, guanidine hydrochloride, HEPESN-(2-hydroxyethyl) piperazineN’-(2-ethanesulfonic acid), N-(2-hydroxyethyl) piperazineN’-(2-ethanesulfonic acid), IPTGisopropyl β-d-1-thiogalactopyranoside, isopropyl β-d-1-thiogalactopyranoside, NMRnuclear magnetic resonance, nuclear magnetic resonance, PMSFphenylmethanesulfonyl fluoride, phenylmethanesulfonyl fluoride, PPRDpercent of predicted disordered residues, percent of predicted disordered residues, TCAtrichloroacetic acid, trichloroacetic acid, Tristris(hydroxymethyl) aminomethane, tris(hydroxymethyl) aminomethane, SDS-PAGEsodium dodecyl sulfate polyacrylamide gel electrophoresis, sodium dodecyl sulfate polyacrylamide gel electrophoresis, S100G, Calmodulin, Calcium sensor, EF-hand, Intrinsic disorder

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Bovine S100 G (calbindin D9k, small Ca2+-binding protein of the EF-hand superfamily) is considered as a calcium buffer protein; i.e., the binding of Ca2+ practically does not change its general conformation. A set of experimental approaches has been used to study structural properties of apo- and Ca2+-loaded forms of mouse S100 G (81.4% identity in amino acid sequence with bovine S100 G). This analysis revealed that, in contrast to bovine S100 G, the removal of calcium ions increases α-helices content of mouse S100 G protein and enhances its accessibility to digestion by α-chymotrypsin. Furthermore, mouse apo-S100 G is characterized by a decreased surface hydrophobicity and reduced tendency for oligomerization. Such behavior is typical of calcium sensor proteins. Apo-state of mouse S100 G still has rather compact structure, which can be cooperatively unfolded by temperature and GdnHCl. Computational analysis of amino acid sequences of S100 G proteins shows that these proteins could be in a disordered state upon a removal of the bound calcium ions. The experimental data show that, although mouse apo-S100 G is flexible compared to the Ca2+-loaded state, the apo-form is not completely disordered and preserves some cooperatively meting structure. The origin of the unexpectedly high stability of mouse S100 G can be rationalized by an exceptionally strong association of its N- and C-terminal parts containing the EF-hands I and II, respectively.

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Cell Calcium, v. 87, art. 102185