Characterization of the Ca²⁺‐switch in skeletal and cardiac muscles

Abstract

To determine the significance of the global structure of the regulatory proteins in the mechanism of the Ca*+-switch in cardiac and skeletal muscle contractions, the properties of a family of Ca Z+-binding proteins with 4 or 3 EF-hand motifs have been studied with desensitized skinned fiber preparations. Proteins with 4 EF hands (such as troponins C -TnCs) are dumb-bell shaped, those with 3 EF hands (parvalbumin) being ellipsoidal. The number of active sites varied between four and two. We find that the ability to anchor in the fiber is limited to proteins with 4 EF hands and, at least, two active Caz+-binding sites, one each in the N-and C-termini. The results suggest that the dumb-bell shaped global structure is critical for the switching action in muscular contraction, and a trigger site in the N-terminus and a structural site in the C-terminus need to be active in order to regulate contractility.

The role of troponin C in the length dependence of Ca(2+)‐sensitive force of mammalian skeletal and cardiac muscles.

“…4. In addition, in 100 mm salt, reconstituted fibres manifest Sr2" sensitivity curves characteristic of cardiac TnC (Babu, Lehman & Gulati, 1989). If cardiac TnC reconstitution had been partial, the force level with maximal activation (with Sr2+ or Ca2+) would have remained submaximal in both low and high salt concentrations.…”

Calcium and strontium activation characteristics of muscle fibres: A comment

“…But, Kerrick and colleagues (1981;1985;Hoar et ai., 1988) had maintained for several years that, in the fibre, the difference between cardiac and skeletal fibres involved complicated proteinprotein interactions outside their regulatory systems. Subsequently, Babu et al (1989) published the converse study where TnC was exchanged in psoas fibres, hoar and colleagues (1988) have now confirmed the results on cardiac muscle. When the indigenous cardiac TnC was replaced with fast skeletal TnC, Sr 2+-and Ca2+-sensitivity values were observed to fall to match the measurements on normal psoas fibre.…”

Central helix role in the contraction‐relaxation switching mechanisms of permeabilized skeletal and smooth muscles with genetic manipulation of calmodulin

“…This was suggested on the basis of results with parvalbumin and oncomodulin [25]. Parvalbumin lacks nearly half of the amino acid structure corresponding to the N-terminus lobe.…”

“…This is associated with a collapsed central helix that folds the protein into an ellipsoidal configuration [1]. Parvalbumin neither binds to TnC-denuded sites in the fiber nor is active in Ca z+ regulation [25]. Oncomodulin is an analog of parvalbumin, but is apparently enzymatically active in solution in vitro [26,27].…”

“…Oncomodulin is an analog of parvalbumin, but is apparently enzymatically active in solution in vitro [26,27]. In the fiber, oncomodulin was inactive like parvalbumin ( [25] on fast-twitch fiber; unpublished results of J. Gulati and A. Babu on smooth muscle), suggesting that the molecule in the shape of an ellipsoid would be unable to enter the attachment sites in vivo. This would indicate also that the diminished activity of the CaM mutant CaMA81-84 in the present study is unlikely to be due to global folding of the molecule, because interaction of the mutant with the target switching-complex in the fiber was normal.…”