© 2003 by European Society of Cardiology
Copyright © 2003, European Society of Cardiology
Myosin light chain isoforms modify force-generating ability of cardiac myosin by changing the kinetics of actin–myosin interaction
aDepartment of Cardiovascular Medicine, Graduate School of Medicine, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8655, Japan
bGraduate School of Frontier Sciences, Institute of Environmental Studies, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan
cDepartment of Physiology, Tsurumi University School of Dental Medicine, 2-3-1 Tsurumi, Tsurumi, Yokohama 230-8501, Japan
dDepartment of Cardiovascular Dynamics, National Cardiovascular Center Research Institute, 5-7-1 Fujishiro-dai, Suita, Osaka, Japan
eDepartment of Physiology, School of Medicine, Teikyo University, 2-11-1 Kaga, Itabashi, Tokyo 173-0003, Japan
*Corresponding author. Tel.: +81-3-3815-5411x33072; fax: +81-3-3814-0021. Email address: hiroyama-tky{at}umin.ac.jp
Objective: To investigate the functional role of myosin light chain (MLC) isoforms in cardiac muscles, we examined the motor function of two different myosins the structure of which differed only in the MLC. Methods: We purified myosin from atria (A-myosin) and ventricles (V-myosin) of young rats, which contained atrial-type and ventricular-type MLCs, respectively, but having identical 
-heavy chain isoform. Actin filament velocity (Vel) was determined in the in vitro motility assay. Average force of myosin molecules (F) was estimated and single events of actin–myosin interaction were recorded with the laser trap technique. Results: Vel was slightly higher in A-myosin than in V-myosin, while actin-activated ATPase activity was not different. F, determined from force versus actin filament length relation, was 
60% higher in V-myosin (3.3 vs. 2.1 pN/µm). The mean duration of isometric force events was longer in V-myosin than in A-myosin (323±13 vs. 294±30 ms, p<0.05), while the amplitudes of unitary displacement and force of a single myosin molecule did not differ between them. Conclusion: The MLC isoform can be a determinant of force-generating ability of cardiac myosin by modulating crossbridge kinetics without affecting the catalytic activity.
KEYWORDS Cardiac myosin; Myosin light chain; ATPase activity; In vitro motility assay; Actin translocating velocity; Laser trap; Unitary displacement; Unitary force
Time for primary review 22 days
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