TAILIEUCHUNG - Báo cáo khoa hoc : Calcium-regulated conformational change in the C-terminal end segment of troponin I and its binding to tropomyosin

The troponin complex plays an essential role in the thin filament regulation of striated muscle contraction. Of the three subunits of troponin, tropo-nin I (TnI) is the actomyosin ATPase inhibitory subunit and its effect is released upon Ca 2+ binding to troponin C. | IFEBS Journal Calcium-regulated conformational change in the C-terminal end segment of troponin I and its binding to tropomyosin Zhiling Zhang1 Shirin Akhter2 Steven Mottl1 and Jian-Ping Jin1 2 1 Evanston Northwestern Healthcare and Northwestern University Evanston IL USA 2 Department of Physiology Wayne State University Schoolof Medicine Detroit MI USA Keywords calcium C-terminus muscle tropomyosin troponin I Correspondence . Jin Department of Physiology Wayne State University Schoolof Medicine Detroit MI 48201 USA Fax 1 313 577 5494 Tel 1 313 577 1520 E-mail jjin@ Received 15 March 2011 revised 26 May 2011 accepted 28 June 2011 doi The troponin complex plays an essential role in the thin filament regulation of striated muscle contraction. Of the three subunits of troponin troponin I TnI is the actomyosin ATPase inhibitory subunit and its effect is released upon Ca2 binding to troponin C. The exon-8-encoded C-terminal end segment represented by the last 24 amino acids of cardiac TnI is highly conserved and is critical to the inhibitory function of troponin. Here we investigated the function and calcium regulation of the C-terminal end segment of TnI. A TnI model molecule was labeled with Alexa Fluor 532 at a Cys engineered at the C-terminal end and used to reconstitute the tertiary troponin complex. A Ca2 -regulated conformational change in the C-ter-minus of TnI was shown by a sigmoid-shape fluorescence intensity titration curve similar to that of the CD calcium titration curve of troponin C. Such corresponding Ca2 responses are consistent with the function of troponin as a coordinated molecular switch. Reconstituted troponin complex containing a mini-troponin T lacking its two tropomyosin-binding sites showed a saturable binding to tropomyosin at pCa 9 but not at pCa 4. This Ca2 -regulated binding was diminished when the C-terminal 19 amino acids of cardiac TnI were removed. These results provided novel evidence

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