Troponin T core structure and the regulatory NH2-terminal variable region.
The conserved central and COOH-terminal regions of troponin T (TnT) interact with troponin C, troponin I, and tropomyosin to regulate striated muscle contraction. Phylogenic data show that the NH2-terminal region has evolved as an addition to the conserved core structure of TnT. This NH2-terminal region does not bind other thin filament proteins, and its sequence is hypervariable between fiber type and developmental isoforms. Previous studies have demonstrated that NH2-terminal modifications alter the COOH-terminal conformation of TnT and thin filament Ca2+-activation, yet the functional core structure of TnT and the mechanism of NH2-terminal modulation are not well understood. To define the TnT core structure and investigate the regulatory role of the NH2-terminal variable region, we investigated two classes of model TnT molecules: (1) NH2-terminal truncated cardiac TnT and (2) chimera proteins consisting of an acidic or basic skeletal muscle TnT NH2-terminus spliced to the cardiac TnT core. Deletion of the TnT hypervariable NH2-terminus preserved binding to troponin I and tropomyosin and sustained cardiac muscle contraction in the heart of transgenic mice. Further deletion of the conserved central region diminished binding to tropomyosin. The reintroduction of differently charged NH2-terminal domains in the chimeric molecules produced long-range conformational changes in the central and COOH-terminal regions to alter troponin I and tropomyosin binding. Similar NH2-terminal charge effects are demonstrated in naturally occurring cardiac TnT isoforms, indicating a physiological significance. These results suggest that the hypervariable NH2-terminal region modulates the conformation and function of the TnT core structure to fine-tune muscle contractility.
Biesiadecki
Brandon J
BJ
Department of Physiology and Biophysics, Case Western Reserve University School of Medicine, 10900 Euclid Avenue, Cleveland, Ohio 44106, USA.
Chong
Stephen M
SM
Nosek
Thomas M
TM
Jin
Jian-Ping
JP
D000595:Amino Acid Sequence; D000818:Animals; D051379:Mice; D008822:Mice, Transgenic; D009119:Muscle Contraction; D009200:Myocardial Contraction; D011485:Protein Binding; D011487:Protein Conformation; D014335:Tropomyosin / Q000378:metabolism; D019209:Troponin C / Q000378:metabolism; D019210:Troponin I / Q000378:metabolism; D020107:Troponin T / Q000737:chemistry* / Q000378:metabolism / Q000502:physiology
D014335:Tropomyosin; D019209:Troponin C; D019210:Troponin I; D020107:Troponin T