Regulation of muscle contraction by Troponin C

It is generally believed that calcium binding to Troponin C (TnC) in striated muscle acts as a simple switch, enabling tropomyosin to unblock myosin-binding sites on actin filaments. TnC has two domains, each containing a pair of structurally homologous sites. The high-affinity Ca2 +/Mg2 + binding sites are believed to have a primarily structural role, anchoring the protein into the troponin complex. The number of available myosin binding sites on the thin filament is determined by the intrinsic properties of the second pair of relatively low-affinity, calcium-specific sites. While TnC is found in all striated muscle, only one functional low-affinity site is common to all types, leading us to believe that it plays a crucial regulatory role in muscle contraction. A series of genetically engineered proteins that modify the calcium-binding properties at site II of the regulatory domain have been introduced into single cell segments of chemically skinned skeletal rabbit psoas muscle fibres. The role of TnC in regulating tension, force development and the maximal rate at which muscle fibers can shorten under zero load, has been investigated. The graded decrease in force with the TnC variants was mutant dependent. Measurements of the maximum shortening velocity using the slack test method produced biphasic plots at pCa 4.0. A graded decrease in force redevelopment (ktr), measured using a quick release with restretch protocol, correlated with the decrease in force. The addition of phosphate to the activating solution decreased force to a greater extent than stiffness and increased kfr. However the early fast phase of the biphasic shortening velocity was unexpectedly slowed in a TnC dependent manner with no increase in the later slow phase of shortening as would be expected if the mutant TnC had simply decreased the level of thin filament activation. Tension transients confirmed that the decrease in tension with the mutant TnC was due to a decrease in the number of bound cross-bridges and not a shift in the force per bridge. This study provides the first evidence that TnC regulates a step in the cross bridge cycle beyond thin filament activation showing that TnC is not just a simple switch.