Key result
Human myosin Vb is a high duty ratio, processive motor with a double-headed structure, where ADP release is the rate-limiting step for its actin-activated ATPase cycle.
This study provides the first mechanoenzymatic characterization of human myosin Vb, demonstrating it is a processive motor with a high duty ratio.
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This biochemical characterization of myosin Vb warrants no clinical changes; it extends motor protein knowledge and leaves open its in vivo roles.
There are three isoforms of class V myosin in mammals. While myosin Va has been studied well, little is known about the function of other myosin V isoforms (Vb and Vc) at a molecular level. Here we report the mechanoenzymatic function of human myosin Vb (HuM5B) for the first time. Electron microscopic observation showed that HuM5B has a double-headed structure with a long neck like myosin Va. V(max) and K(actin) of the actin-activated ATPase activity of HuM5B were 9.7 +/- 0.4 s(-)(1) and 8.5 +/- 0.1 microM, respectively. K(actin) and K(ATP) of the actin-activated ATPase activity were significantly higher than those of myosin Va. ADP markedly inhibited the ATPase activity. The rate of release of ADP from acto-HuM5B was 12.2 +/- 0.5 s(-)(1), which was comparable to the V(max) of the actin-activated ATPase activity. These results suggest that ADP release is the rate-limiting step for the actin-activated ATPase cycle; thus, HuM5B is a high duty ratio myosin. Consistently, the actin gliding velocity (0.22 +/- 0.03 microm/s) remained constant at a low motor density. The actin filament landing assay revealed that a single HuM5B molecule is sufficient to move the actin filament continuously, indicating that HuM5b is a processive motor.
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Watanabe et al. (2006) studied this question. Human myosin Vb (HuM5B) vs. Myosin Va was evaluated on Actin-activated ATPase activity and mechanoenzymatic function. Human myosin Vb is a high duty ratio, processive motor with a double-headed structure, where ADP release is the rate-limiting step for its actin-activated ATPase cycle.
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