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J. Biol. Chem., Vol. 276, Issue 19, 15753-15760, May 11, 2001

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A Proteolytic NH₂-terminal Truncation of Cardiac Troponin I That Is Up-regulated in Simulated Microgravity^*

Zhi-Bin Yu^§, Li-Fan Zhang^§, and Jian-Ping Jin^¶

From the  Department of Physiology and Biophysics, Case Western Reserve University School of Medicine, Cleveland, Ohio 44106-4970 and the ^§ Department of Aerospace Physiology, The Fourth Military Medical University, Xi'an, China 710032

In a tail suspension rat model, we investigated changes in myofilament protein during cardiac adaptation in simulated microgravity. Contractile force and velocity of cardiac muscle were decreased in the tail suspension rats as compared with the control. Ca²⁺-dependent actomyosin ATPase activity was also decreased; however, sensitivity of cardiac muscle to Ca²⁺ activation was unchanged. There was no change in expression of myosin heavy chain, tropomyosin, troponin T, or troponin I isoforms in hearts of tail suspension rats. A novel finding is a fragment of cardiac troponin I (cTnI) that had increased amounts in the heart of tail suspension rats. Binding of this cTnI fragment by a monoclonal antibody that specifically recognizes the COOH terminus indicates an intact COOH terminus. NH₂-terminal sequence analysis of the cTnI fragment revealed truncations primarily of amino acids 1-26 and 1-27 and smaller amounts of 1-30, including Ser²³ and Ser²⁴, which are substrates of protein kinase A phosphorylation. This cTnI fragment is present in normal cardiac muscle and incorporated into myofibrils, indicating a role in regulating contractility. This proteolytic modification of cTnI up-regulated during simulated microgravity suggests a potential role of the NH₂-terminal segment of cTnI in functional adaptations of cardiac muscle.

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