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J. Biol. Chem., Vol. 283, Issue 23, 15611-15618, June 6, 2008

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Smooth Muscle Myosin Phosphorylated at Single Head Shows Sustained Mechanical Activity^*

Hiroto Tanaka¹, Kazuaki Homma^¶1, Howard D. White^||, Toshio Yanagida^**, and Mitsuo Ikebe^¶2

From the PRESTO, Japan Science and Technology Agency, 3, 5 Chiyodaku, Tokyo 102-0075, Japan, IMRAM, Tohoku University, 1-1 Katahira, 2-Chome, Aobaku, Sendai 980-8577, Japan, the ^¶Department of Physiology, University of Massachusetts Medical School, Worcester, Massachusetts 01655-0127, the ^||Department of Physiological Sciences, Eastern Virginia Medical School, Norfolk, Virginia 23507-1980, and the ^**Soft Biosystem Group, Laboratories for Nanobiology, Graduate School of Frontier Biosciences, Osaka University, 1-3 Yamadaoka, Suita, Osaka 565-0871, Japan

Smooth muscle contraction is regulated by the phosphorylation of myosin. It is well known that tonic smooth muscles can maintain force with low energy consumption (latch state); however, the molecular mechanism underlying this phenomenon is unresolved. Here we show that single-head phosphorylated smooth myosin (SHPMII) exhibits fast (24 s^–1) and slow prolonged (1 s^–1) actin interactions, whereas double-head phosphorylated myosin (DHPMII) predominantly exhibits the fast (29 s^–1) interaction, suggesting that the phosphorylated head of SHPMII is mechanically as active as that of DHPMII. Both the fast and the slow actin interactions of SHPMII support the positive net mechanical displacement of actin. The actin translocating velocity of SHPMII was much slower than that of DHPMII, which is consistent with the slow actin interaction of SHPMII. We propose that the "latch state" can be explained by the motor characteristics of SHPMII that is present during the sustained phase of contraction.

Received for publication, December 31, 2007 , and in revised form, April 9, 2008.

^* This work was supported, in whole or in part, by National Institutes of Health Grants AR41653 and HL073050. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplemental Figs. S1–S3, supplemental Tables S1 and S2, supplemental Movies S1–S4, and supplemental text.

¹ These authors contributed equally to this work.

² To whom correspondence should be addressed. E-mail: mitsuo.ikebe{at}umassmed.edu.

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