Influence of enhanced troponin C Ca2+-binding affinity on cooperative thin filament activation in rabbit skeletal muscle.
We studied how enhanced skeletal troponin C (sTnC) Ca2+-binding affinity affects cooperative thin filament activation and contraction in single demembranated rabbit psoas fibres. Three sTnC mutants were created and incorporated into skeletal troponin (sTn) for measurement of Ca2+ dissociation, resulting in the following order of rates: wild-type (WT) sTnC-sTn>sTnC(F27W)-sTn>M80Q sTnC-sTn>M80Q sTnCF27W-sTn. Reconstitution of sTnC-extracted fibres increased Ca2+ sensitivity of steady-state force (pCa(50)) by 0.08 for M80Q sTnC, 0.15 for sTnCF27W and 0.32 for M80Q sTnCF27W with minimal loss of slope (nH, degree of cooperativity). Near-neighbour thin filament regulatory unit (RU) interactions were reduced in fibres by incorporating mixtures of WT or mutant sTnC and D28A, D64A sTnC (xxsTnC) that does not bind Ca2+ at N-terminal sites. Reconstitution with sTnC: xxsTnC mixtures to 20% of pre-exchanged maximal force reduced pCa50 by 0.35 for sTnC: xxsTnC, 0.25 for M80Q sTnC: xxsTnC, and 0.10 for M80Q sTnCF27W: xxsTnC. It is interesting that pCa50 increased by approximately 0.1 for M80Q sTnC and approximately 0.3 for M80Q sTnCF27W when near-neighbour RU interactions were reduced; these values are similar in magnitude to those for fibres reconstituted with 100% mutant sTnC. After reconstitution with sTnC: xxsTnC mixtures, nH decreased to a similar value for all mutant sTnCs. Altered sTnC Ca2+-binding properties (M80Q sTnCF27W) did not affect strong crossbridge inhibition by 2,3-butanedione monoxime when near-neighbour thin filament RU interactions were reduced. Together these results suggest increased sTnC Ca2+ affinity strongly influences Ca2+ sensitivity of steady-state force without affecting near-neighbour thin filament RU cooperative activation or the relative contribution of crossbridges versus Ca2+ to thin filament activation.
Kreutziger
Kareen L
KL
Department of Bioengineering, Box 355061, University of Washington, Seattle, WA 98195, USA.
Gillis
Todd E
TE
Davis
Jonathan P
JP
Tikunova
Svetlana B
SB
Regnier
Michael
M
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D003931:Diacetyl / Q000031:analogs & derivatives / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D008297:Male; D009119:Muscle Contraction / Q000502:physiology; D018485:Muscle Fibers, Skeletal / Q000187:drug effects / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D009154:Mutation / Q000235:genetics; D011485:Protein Binding; D011817:Rabbits; D019209:Troponin C / Q000235:genetics / Q000378:metabolism*
D004791:Enzyme Inhibitors; D019209:Troponin C; C004717:diacetylmonoxime; D003931:Diacetyl; D002118:Calcium