Cardiac troponin T isoforms affect the Ca2+ sensitivity and inhibition of force development. Insights into the role of troponin T isoforms in the heart.
At least four isoforms of troponin T (TnT) exist in the human heart, and they are expressed in a developmentally regulated manner. To determine whether the different N-terminal isoforms are functionally distinct with respect to structure, Ca(2+) sensitivity, and inhibition of force development, the four known human cardiac troponin T isoforms, TnT1 (all exons present), TnT2 (missing exon 4), TnT3 (missing exon 5), and TnT4 (missing exons 4 and 5), were expressed, purified, and utilized in skinned fiber studies and in reconstituted actomyosin ATPase assays. TnT3, the adult isoform, had a slightly higher alpha-helical content than the other three isoforms. The variable region in the N terminus of cardiac TnT was found to contribute to the determination of the Ca(2+) sensitivity of force development in a charge-dependent manner; the greater the charge the higher the Ca(2+) sensitivity, and this was primarily because of exon 5. These studies also demonstrated that removal of either exon 4 or exon 5 from TnT increased the cooperativity of the pCa force relationship. Troponin complexes reconstituted with the four TnT isoforms all yielded the same maximal actin-tropomyosin-activated myosin ATPase activity. However, troponin complexes containing either TnT1 or TnT2 (both containing exon 5) had a reduced ability to inhibit this ATPase activity when compared with wild type troponin (which contains TnT3). Interestingly, fibers containing these isoforms also showed less relaxation suggesting that exon 5 of cardiac TnT affects the ability of Tn to inhibit force development and ATPase activity. These results suggest that the different N-terminal TnT isoforms would produce different functional properties in the heart that would directly affect myocardial contraction.
Gomes
Aldrin V
AV
Department of Molecular and Cellular Pharmacology, University of Miami School of Medicine, Miami, Florida 33101, USA.
Guzman
Georgianna
G
Zhao
Jiaju
J
Potter
James D
JD
D000595:Amino Acid Sequence; D001483:Base Sequence; D002118:Calcium / Q000378:metabolism*; D002942:Circular Dichroism; D017931:DNA Primers; D006321:Heart / Q000502:physiology*; D006801:Humans; D008969:Molecular Sequence Data; D016296:Mutagenesis; D009206:Myocardium / Q000378:metabolism*; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D017386:Sequence Homology, Amino Acid; D013056:Spectrophotometry, Ultraviolet; D020107:Troponin T / Q000235:genetics / Q000378:metabolism / Q000502:physiology*
D017931:DNA Primers; D020033:Protein Isoforms; D020107:Troponin T; D002118:Calcium