Figure 4.

Influence of Tn subunits on myofibrillar ATPase activity and labeled Tn exchange pattern. Intrinsic Tn (TnCIT) was exchanged out with either TnIT or TnT at pCa 4. ATPase activity was measured at pCa 9 and pCa 4 and expressed relative to TnCIT myofibril pCa 4 activity (a). Labeled Tn was exchanged into these myofibrils as for Figure 2 for 8 min (b). Images are presented as in Figure 2. Exchange of TnIT for TnCIT resulted in complete loss of calcium activation of ATPase activity while exchange of TnT for TnCIT resulted in complete loss of calcium regulation of ATPase activity. There was limited exchange in the non-overlap region for TnIT myofibrils showing that TnC was needed for the calcium dependency of Tn exchange (C state). For TnT myofibrils, non-overlap region exchange (C state) was fast, independent of calcium showing that TnI was needed for B state and TnIC was needed for calcium dependency of switching between the B and C state. Another feature of TnT myofibrils was that exchange was slower in the overlap than non-overlap, suggesting that strong cross-bridges (M state) slowed TnT dissociation.