Solution structure of the Apo C-terminal domain of the Lethocerus F1 troponin C isoform.
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.
De Nicola
Gian Felice
GF
Molecular Structure Division, National Institute for Medical Research, MRC, The Ridgeway, Mill Hill, London NW71AA, UK.
Martin
Stephen
S
Bullard
Belinda
B
Pastore
Annalisa
A
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
D019476:Insect Proteins; D020033:Protein Isoforms; D019209:Troponin C; D019210:Troponin I; D002118:Calcium