Thin-filament-binding domains of cardiac and fast skeletal muscle troponin I isoforms as studied by epitope tagging.
We examined the binding domains of cardiac and fast skeletal muscle troponin I (CTnI and FTnI, respectively) to myofibrils (MFs). Deletion mutants containing CTnI amino acid residues 1-79, 43-207 and 80-207 (CTnI-head, CTnI-tail-I and CTnI-tail-2, respectively) and FTnI amino acid residues 1-54 and 55-182 (FTnI-head and FTnI-tail, respectively) were transiently expressed in cardiac and fast skeletal muscle cells. To monitor the intracellular localization of these exogenously introduced truncated TnIs, epitope tagging was used. CTnI-tail-1 was incorporated into cardiac MFs specifically, but CTnI-tail-2 was not assembled onto any MFs examined. This suggests that there is no potent actin filament-binding site in CTnI-tail-2. Since CTnI-tail-1 has an amino acid extension (CTnI residues 43-79) whose sequence is longer than that of CTnI-head-2; it appears that this sequence extension is important in binding to cardiac MFs. FTnI-tail, containing the inhibitory domain of actomyosin ATPase, showed intensive and specific incorporation into fast MFs. FTnI-tail was a homologous fragment of CTnI-tail-2, but the binding patterns of these two domains differed greatly from each other. It is possible that the absence of potent binding affinity of CTnI-tail-2 corresponding to the inhibitory domain of actomyosin ATPase is advantageous for continuous cardiac muscle contraction, since a potent inhibitory activity is a serious obstacle to cardiac muscle contraction. It can be assumed that distinctive binding ability of functional domains of TnI-tails reflect unique adaptations to muscles with different physiological properties.
Toyota
N
N
Department of Anatonmy/Cell Biology, Chiba University, Japan. toyota@med.m.chiba-u.ac.jp
Uzawa
H
H
Komiyama
M
M
Shimada
Y
Y
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D015153:Blotting, Western; D002478:Cells, Cultured; D002645:Chickens; D004247:DNA / Q000235:genetics; D018604:Epitope Mapping; D005455:Fluorescent Antibody Technique; D017353:Gene Deletion; D008969:Molecular Sequence Data; D018656:Muscle Fibers, Fast-Twitch / Q000737:chemistry; D018482:Muscle, Skeletal / Q000737:chemistry*; D009154:Mutation; D009206:Myocardium / Q000737:chemistry*; D015345:Oligonucleotide Probes; D020033:Protein Isoforms / Q000032:analysis; D014162:Transfection; D019210:Troponin I / Q000032:analysis* / Q000737:chemistry / Q000235:genetics
D015345:Oligonucleotide Probes; D020033:Protein Isoforms; D019210:Troponin I; D004247:DNA