A troponin switch that regulates muscle contraction by stretch instead of calcium.
The flight muscles of many insects have a form of regulation enabling them to contract at high frequencies. The muscles are activated by periodic stretches at low Ca2+ levels. The same muscles also give isometric contractions in response to higher Ca2+. We show that the two activities are controlled by different isoforms of TnC (F1 and F2) within single myofibrils. F1 binds one Ca2+ with high affinity in the C-terminal domain and F2 binds one Ca2+ in the C-terminal domain and one exchangeable Ca2+ in the N-terminal domain. We have characterised the isoforms and determined their effect on the development of stretch-activated and Ca2+-activated tension by replacing endogenous TnC in Lethocerus flight muscle fibres with recombinant isoforms. Fibres with F1 gave stretch-activated tension and minimal isometric tension; those with F2 gave Ca2+-dependent isometric tension and minimal stretch-activated tension. Regulation by a TnC responding to stretch rather than Ca2+ is unprecedented and has resulted in the ability of insect flight muscle to perform oscillatory work at low Ca2+ concentrations, a property to which a large number of flying insects owe their evolutionary success.
Agianian
Bogos
B
European Molecular Biology Laboratory, Meyerhofstrasse 1, Heidelberg, Germany.
Krzic
Uros
U
Qiu
Feng
F
Linke
Wolfgang A
WA
Leonard
Kevin
K
Bullard
Belinda
B
D000205:Actomyosin / Q000378:metabolism; D000251:Adenosine Triphosphatases / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000502:physiology*; D005426:Flight, Animal; D020062:Heteroptera / Q000378:metabolism / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology; D009210:Myofibrils / Q000378:metabolism / Q000502:physiology*; D020033:Protein Isoforms / Q000378:metabolism; D019209:Troponin C / Q000378:metabolism*
D020033:Protein Isoforms; D019209:Troponin C; D000205:Actomyosin; D000251:Adenosine Triphosphatases; D002118:Calcium