| 1. |
- Bauer, Mikael, et al.
(författare)
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Zn2+ binding to human calbindin D(28k) and the role of histidine residues.
- 2008
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Ingår i: Protein Science. - : Wiley. - 1469-896X .- 0961-8368. ; 17:4, s. 760-767
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the binding of Zn2+ to the hexa EF-hand protein, calbindin D(28k)-a strong Ca2+-binder involved in apoptosis regulation-which is highly expressed in brain tissue. By use of radioblots, isothermal titration calorimetry, and competition with a fluorescent Zn2+ chelator, we find that calbindin D(28k) binds Zn2+ to three rather strong sites with dissociation constants in the low micromolar range. Furthermore, we conclude based on spectroscopic investigations that the Zn2+-bound state is structurally distinct from the Ca2+-bound state and that the two forms are incompatible, yielding negative allosteric interaction between the zinc- and calcium-binding events. ANS titrations reveal a change in hydrophobicity upon binding Zn2+. The binding of Zn2+ is compatible with the ability of calbindin to activate myo-inositol monophosphatase, one of the known targets of calbindin. Through site-directed mutagenesis, we address the role of cysteine and histidine residues in the binding of Zn2+. Mutation of all five cysteines into serines has no effect on Zn2+-binding affinity or stoichiometry. However, mutating histidine 80 into a glutamine reduces the binding affinity of the strongest Zn2+ site, indicating that this residue is involved in coordinating the Zn2+ ion in this site. Mutating histidines 5, 22, or 114 has significantly smaller effects on Zn2+-binding affinity.
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| 2. |
- Carey, Jannette, et al.
(författare)
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Protein reconstitution and three-dimensional domain swapping: Benefits and constraints of covalency
- 2007
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Ingår i: Protein Science. - : Wiley. - 1469-896X .- 0961-8368. ; 16:11, s. 2317-2333
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Forskningsöversikt (refereegranskat)abstract
- The phenomena of protein reconstitution and three-dimensional domain swapping reveal that highly similar structures can be obtained whether a protein is comprised of one or more polypeptide chains. In this review, we use protein reconstitution as a lens through which to examine the range of protein tolerance to chain interruptions and the roles of the primary structure in related features of protein structure and folding, including circular permutation, natively unfolded proteins, allostery, and amyloid fibril formation. The results imply that noncovalent interactions in a protein are sufficient to specify its structure under the constraints imposed by the covalent backbone.
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| 3. |
- Fast, Jonas, et al.
(författare)
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Stability of HAMLET--A kinetically trapped {alpha}-lactalbumin oleic acid complex.
- 2005
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Ingår i: Protein Science. - : Wiley. - 1469-896X .- 0961-8368. ; 14:2, s. 329-340
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Tidskriftsartikel (refereegranskat)abstract
- The stability toward thermal and urea denaturation was measured for HAMLET (human -lactalbumin made lethal to tumor cells) and -lactalbumin, using circular dichroism and fluorescence spectroscopy as well as differential scanning calorimetry. Under all conditions examined, HAMLET appears to have the same or lower stability than -lactalbumin. The largest difference is seen for thermal denaturation of the calcium free (apo) forms, where the temperature at the transition midpoint is 15°C lower for apo HAMLET than for apo -lactalbumin. The difference becomes progressively smaller as the calcium concentration increases. Denaturation of HAMLET was found to be irreversible. Samples of HAMLET that have been renatured after denaturation have lost the specific biological activity toward tumor cells. Three lines of evidence indicate that HAMLET is a kinetic trap: (1) It has lower stability than -lactalbumin, although it is a complex of -lactalbumin and oleic acid; (2) its denaturation is irreversible and HAMLET is lost after denaturation; (3) formation of HAMLET requires a specific conversion protocol.
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| 4. |
- Lindman, Stina, et al.
(författare)
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Green fluorescence induced by EF-hand assembly in a split GFP system.
- 2009
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Ingår i: Protein Science. - : Wiley. - 1469-896X .- 0961-8368. ; 18:6, s. 1221-1229
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Tidskriftsartikel (refereegranskat)abstract
- The affinity between the 1-157 and 158-238 fragments of green fluorescent protein (GFP) is too low for spontaneous in vivo reassembly of the protein upon co-expression of the two fragments. This prevents chromophore maturation and the cells lack GFP fluorescence. We have utilized the very high affinity between the two EF-hands of calbindin D(9k) to facilitate GFP assembly from its fragments and to introduce a calcium dependent molecular switch. In GFPN-EF1, residues 1-157 of GFP are fused to residues 1-43 of calbindin, and in EF2-GFPC, residues 44-75 of calbindin are fused to residues 158-238 of GFP. When co-expressed, GFPN-EF1 and EF2-GFPC associate spontaneously and rapidly resulting in a folded reconstituted protein with bright GFP fluorescence. The high affinity of GFPN-EF1 for EF2-GFPC leads to brighter fluorescence of the cells compared to cells with a control constructs carrying leucine zippers (Wilson et al., Nature Methods 2004;3:255). The complex of GFPN-EF1 and EF2-GFPC was purified from cells using metal-ion chelate chromatography and the temperature dependence of GFP fluorescence was found to be calcium dependent. The GFPN-EF1 and EF2-GFPC fragments were separated by ion exchange chromatography. The assembly of the fragments was found to be reversible and the complex was regained upon mixing, as evidenced by surface plasmon resonance (SPR) data. The affinity between GFPN-EF1 and EF2-GFPC as well as rates of association and dissociation were found to be Ca(2+)-dependent.
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| 5. |
- Palczewska, M, et al.
(författare)
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Characterization of calretinin I-II as an EF-hand, Ca2+, H+-sensing domain
- 2005
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Ingår i: Protein Science. - : Wiley. - 1469-896X .- 0961-8368. ; 14:7, s. 1879-1887
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Tidskriftsartikel (refereegranskat)abstract
- Calretinin, a neuronal protein with well-defined calcium-binding properties, has a poorly defined function. The pH dependent properties of calretinin (CR), the N-terminal (CR I-II), and C-terminal (CR III-VI) domains were investigated. A drop in pH within the intracellular range (from pH 7.5 to pH 6.5) leads to an increased hydrophobicity of calcium-bound CR and its domains as reported by fluorescence spectroscopy with the hydrophobic probe 2-(p-toluidino)-6-naphthalenesulfonic acid (TNS). The TNS data for the N- and C-terminal domains of CR are additive, providing further support for their independence within the full-length protein. Our work concentrated on CR I-II, which was found to have hydrophobic properties similar to calmodulin at lower pH. The elution of CR I-II from a phenyl-Sepharose column was consistent with the TNS data. The pH-dependent structural changes were further localized to residues 13-28 and 44-51 using nuclear magnetic resonance spectroscopy chemical shift analysis, and there appear to be no large changes in secondary structure. Protonation of His12 and/or His27 side chains, coupled with calcium chelation, appears to lead to the organization of a hydrophobic pocket in the N-terminal domain. CR may sense and respond to calcium, proton, and other signals, contributing to conflicting data on the proteins role as a calcium sensor or calcium buffer.
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| 6. |
- Svensson, Malin, et al.
(författare)
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alpha-Lactalbumin unfolding is not sufficient to cause apoptosis, but is required for the conversion to HAMLET (human alpha-lactalbumin made lethal to tumor cells).
- 2003
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Ingår i: Protein Science. - : Wiley. - 1469-896X .- 0961-8368. ; 12:12, s. 2794-2804
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Tidskriftsartikel (refereegranskat)abstract
- HAMLET (human -lactalbumin made lethal to tumor cells) is a complex of human -lactalbumin and oleic acid (C18:1:9 cis) that kills tumor cells by an apoptosis-like mechanism. Previous studies have shown that a conformational change is required to form HAMLET from -lactalbumin, and that a partially unfolded conformation is maintained in the HAMLET complex. This study examined if unfolding of -lactalbumin is sufficient to induce cell death. We used the bovine -lactalbumin Ca2+ site mutant D87A, which is unable to bind Ca2+, and thus remains partially unfolded regardless of solvent conditions. The D87A mutant protein was found to be inactive in the apoptosis assay, but could readily be converted to a HAMLET-like complex in the presence of oleic acid. BAMLET (bovine -lactalbumin made lethal to tumor cells) and D87A-BAMLET complexes were both able to kill tumor cells. This activity was independent of the Ca2+site, as HAMLET maintained a high affinity for Ca2+ but D87A-BAMLET was active with no Ca2+ bound. We conclude that partial unfolding of -lactalbumin is necessary but not sufficient to trigger cell death, and that the activity of HAMLET is defined both by the protein and the lipid cofactor. Furthermore, a functional Ca2+-binding site is not required for conversion of -lactalbumin to the active complex or to cause cell death. This suggests that the lipid cofactor stabilizes the altered fold without interfering with the Ca2+site.
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| 7. |
- Svensson, Malin, et al.
(författare)
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Lipids as cofactors in protein folding: Stereo-specific lipid-protein interactions are required to form HAMLET (human alpha-lactalbumin made lethal to tumor cells).
- 2003
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Ingår i: Protein Science. - : Wiley. - 1469-896X .- 0961-8368. ; 12:12, s. 2805-2814
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Tidskriftsartikel (refereegranskat)abstract
- Proteins can adjust their structure and function in response to shifting environments. Functional diversity is created not only by the sequence but by changes in tertiary structure. Here we present evidence that lipid cofactors may enable otherwise unstable protein folding variants to maintain their conformation and to form novel, biologically active complexes. We have identified unsaturated C18 fatty acids in the cis conformation as the cofactors that bind apo -lactalbumin and form HAMLET (human -lactalbumin made lethal to tumor cells). The complexes were formed on an ion exchange column, were stable in a molten globule-like conformation, and had attained the novel biological activity. The protein–fatty acid interaction was specific, as saturated C18 fatty acids, or unsaturated C18:1trans conformers were unable to form complexes with apo -lactalbumin, as were fatty acids with shorter or longer carbon chains. Unsaturated cis fatty acids other than C18:1:9cis were able to form stable complexes, but these were not active in the apoptosis assay. The results demonstrate that stereo-specific lipid–protein interactions can stabilize partially unfolded conformations and form molecular complexes with novel biological activity. The results offer a new mechanism for the functional diversity of proteins, by exploiting lipids as essential, tissue-specific cofactors in this process.
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| 8. |
- Vanbelle, Christophe, et al.
(författare)
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Deamidation and disulfide bridge formation in human calbindin D-2gk with effects on calcium binding
- 2005
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Ingår i: Protein Science. - : Wiley. - 1469-896X .- 0961-8368. ; 14:4, s. 968-979
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Tidskriftsartikel (refereegranskat)abstract
- Calbindin D-28k (calbindin) is a cytoplasmic protein expressed in the central nervous system, which is implied in Ca2+ homeostasis and enzyme regulation. A combination of biochemical methods and mass spectrometry has been used to identify post-translational modifications of human calbindin. The protein was studied at 37 degrees C or 50 degrees C in the presence or absence of Ca2+. One deamidation site was identified at position 203 (Asn) under all conditions. Kinetic experiments show that deamidation of Asn 203 occurs at a rate of 0.023 h(-1) at 50 degrees C for Ca2+-free calbindin. Deamidation is slower for the Ca2+-saturated protein. The deamidation process leads to two Asp iso-forms, regular Asp and iso-Asp. The form with regular Asp 203 binds four Ca2+ ions with high affinity and positive cooperativity, i.e., in a very similar manner to non-deamidated protein. The form with beta-aspartic acid (or iso-Asp 203) has reduced affinity for two or three sites leading to sequential Ca2+ binding, i.e., the Ca2+-binding properties are significantly perturbed. The status of the cysteine residues was also assessed. Under nonreducing conditions, cysteines 94 and 100 were found both in reduced and oxidized form, in the latter case in an intramolecular disulfide bond. In contrast, cysteines 187, 219, and 257 were not involved in any disulfide bonds. Both the reduced and oxidized forms of the protein bind four Ca2+ ions with high affinity in a parallel manner and with positive cooperativity.
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