Molecular biology of the length-tension relation in cardiac muscle

TnC in the thin filament is found to act as the putative length-sensor in cardiac muscle. Techniques of genetic engineering and TnC-exchange in skinned fibers are combined to delineate the molecular principles governing the length induced modulations in TnC. Cardiac TnC is a better length-sensor than skeletal TnC, and the differences in the two moieties are used as a guide to explore the length-sensing mechanism. Studies are described with mutants of both cardiac and skeletal TnCs: (i) a mutant of cardiac TnC in which Ca(2+)-binding to site 1 was restored (made by John Putkey), and (ii) two mutants of skeletal TnC, (a) with site 1 inactivated, and (b) a chimera, in which 41 residues in the N-terminus are entirely of the cardiac type and the remaining residues are all of the skeletal type (made at Albert Einstein). The results indicate that Ca(2+)-binding property of site 1 is only partly responsible for the length-sensitivity of TnC, and that the residues outside loop-1 are also critical in both the on-off switching mechanism as well as the length-sensing mechanism. Further, we have also found that thin filaments are polarized such that Ca(2+)-affinity may be high at the free end of the filament and decreases towards the Z-line, reaching its lowest value at the fixed end. This could serve as the basis for the conversion of sarcomere length change into a signal that can be transduced from the Z-lines to the TnC moiety in actin filaments.