title: Solution structure of the Apo C-terminal domain of the Lethocerus F1 troponin C isoform.
abstract: Muscle contraction is activated by two distinct mechanisms. One depends on the calcium influx, and the other is calcium-independent and activated by mechanical stress. A prototypical example of stretch activation is observed in insect muscles. In Lethocerus, a model system ideally suited for studying stretch activation, the two mechanisms seem to be under the control of different isoforms of troponin C (TnC), F1 and F2, which are responsible for stretch and calcium-dependent regulation, respectively. We have previously shown that F1 TnC is a typical collapsed dumbbell EF-hand protein that accommodates one calcium ion in its fourth EF-hand. When calcium loaded, the C-terminal domain of F1 TnC is in an open conformation which allows binding to troponin I. We have determined the solution structure of the isolated F1 TnC C-terminal domain in the absence of calcium and have compared it together with its dynamical properties with those of the calcium-loaded form. The domain is folded also in the absence of calcium and is in a closed conformation. Binding of a single calcium is sufficient to induce a modest but clear closed-to-open conformational transition and releases the conformational entropy observed in the calcium-free form. These results provide the first example of a TnC domain in which the presence of only one calcium ion is sufficient to induce a closed-to-open transition and clarify the role of calcium in stretch activation.
authors.affiliation: Molecular Structure Division, National Institute for Medical Research, MRC, The Ridgeway, Mill Hill, London NW71AA, UK.
authors.forename: Gian Felice
authors.initials: GF
authors.lastname: De Nicola
authors.forename: Stephen
authors.initials: S
authors.lastname: Martin
authors.forename: Belinda
authors.initials: B
authors.lastname: Bullard
authors.forename: Annalisa
authors.initials: A
authors.lastname: Pastore
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D002942:Circular Dichroism; D020062:Heteroptera / Q000378:metabolism*; D019476:Insect Proteins / Q000737:chemistry* / Q000378:metabolism*; D018482:Muscle, Skeletal / Q000378:metabolism; D019906:Nuclear Magnetic Resonance, Biomolecular; D011485:Protein Binding; D020033:Protein Isoforms / Q000737:chemistry* / Q000378:metabolism*; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D019209:Troponin C / Q000737:chemistry* / Q000378:metabolism*; D019210:Troponin I / Q000378:metabolism
chemical_list: D019476:Insect Proteins; D020033:Protein Isoforms; D019209:Troponin C; D019210:Troponin I; D002118:Calcium