A proteolytic NH2-terminal truncation of cardiac troponin I that is up-regulated in simulated microgravity.
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. Ca(2+)-dependent actomyosin ATPase activity was also decreased; however, sensitivity of cardiac muscle to Ca(2+) 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. NH(2)-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 Ser(23) and Ser(24), 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 NH(2)-terminal segment of cTnI in functional adaptations of cardiac muscle.
Yu
Z B
ZB
Department of Physiology and Biophysics, Case Western Reserve University School of Medicine, Cleveland, Ohio 44106-4970, USA.
Zhang
L F
LF
Jin
J P
JP
D000199:Actins / Q000378:metabolism; D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism; D004558:Electric Stimulation; D005786:Gene Expression Regulation; D006352:Heart Ventricles; D019417:Hindlimb Suspension; D066298:In Vitro Techniques; D008297:Male; D008969:Molecular Sequence Data; D009200:Myocardial Contraction / Q000502:physiology; D009206:Myocardium / Q000378:metabolism; D009210:Myofibrils / Q000502:physiology; D018995:Myosin Heavy Chains / Q000235:genetics; D010210:Papillary Muscles / Q000502:physiology*; D010446:Peptide Fragments / Q000737:chemistry / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D012694:Serine; D019210:Troponin I / Q000737:chemistry / Q000235:genetics* / Q000502:physiology*; D018474:Weightlessness Simulation
D000199:Actins; D010446:Peptide Fragments; D019210:Troponin I; D012694:Serine; D017868:Cyclic AMP-Dependent Protein Kinases; D018995:Myosin Heavy Chains