Expression, zinc-affinity purification, and characterization of a novel metal-binding cluster in troponin T: metal-stabilized alpha-helical structure and effects of the NH2-terminal variable region on the conformation of intact troponin T and its association with tropomyosin.
A repeating metal-binding (Cu2+ > Ni2+ > Zn2+ approximately Co2+) sequence [HE/AEAH]4 (Tx) has been recently identified in the NH2-terminal variable region of troponin T (TnT) isoforms specifically expressed in the breast but not leg muscles of the avian orders of Galliformes and Craciformes [Jin, J.-P., & Smillie, L. B. (1994) FEBS Lett. 341, 135-140]. In the present study, two expression plasmids were constructed to produce chicken TnT1 NH2-terminal fragments of 47 (N47) or 165 (N165) amino acids containing the Tx metal-binding cluster. The recombinant protein/peptide was expressed in Escherichia coli BL21(DE3)pLysS and purified by a highly effective Zn(2+)-affinity chromatography method. Amino acid analyses, NH2-terminal peptide sequencing, mass spectrometry and immunological identification confirmed the authenticity of the genetically engineered TnT fragments. In the presence of 2,2,2-trifluoroethanol, transition metals had significant effects on the secondary structure of TnT fragment N47, as shown by circular dichroism. N165 in non-denaturing buffer demonstrated alpha-helical content comparable to previous data from rabbit fast skeletal TnT fragment T1. Zn(2+)-binding avidity of the metal-binding TnT and its fragments demonstrated tertiary relationships between the NH2-terminal variable region and the COOH-terminal segment of the intact TnT protein. Solid-phase protein-binding assays established that Zn(2+)-binding to the Tx cluster induces epitopic structure changes in this NH2-terminal segment, further affecting other epitopic structures of intact TnT as well as the function of TnT's tropomyosin binding-sites. The results demonstrate that metal ion-binding to the Tx cluster reconfigures the overall conformation of TnT through structural relationships between the NH2-terminal variable region and other domains of the intact TnT molecule. Accordingly, the developmental and/or muscle type specific NH2-terminal structure of TnT isoforms may modulate the Ca(2+)-activation of muscle contraction.
Ogut
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Department of Medical Biochemistry, University of Calgary Faculty of Medicine, Alberta, Canada.
Jin
J P
JP
D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001665:Binding Sites; D002645:Chickens; D002846:Chromatography, Affinity; D018076:DNA, Complementary / Q000235:genetics; D000939:Epitopes / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D004926:Escherichia coli / Q000235:genetics; D015870:Gene Expression; D066298:In Vitro Techniques; D008670:Metals / Q000378:metabolism / Q000494:pharmacology; D008969:Molecular Sequence Data; D015394:Molecular Structure; D010446:Peptide Fragments / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D011487:Protein Conformation; D017433:Protein Structure, Secondary / Q000187:drug effects; D011817:Rabbits; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D014335:Tropomyosin / Q000378:metabolism; D014336:Troponin / Q000737:chemistry* / Q000235:genetics* / Q000378:metabolism; D020107:Troponin T; D015032:Zinc / Q000378:metabolism / Q000494:pharmacology
D018076:DNA, Complementary; D000939:Epitopes; D008670:Metals; D010446:Peptide Fragments; D011994:Recombinant Proteins; D014335:Tropomyosin; D014336:Troponin; D020107:Troponin T; D015032:Zinc