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First published online November 30, 2007
Journal of Experimental Biology 210, 4399-4410 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.008722
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Unphosphorylated twitchin forms a complex with actin and myosin that may contribute to tension maintenance in catch

Daisuke Funabara1,*, Chieko Hamamoto2,*, Koji Yamamoto1, Akinori Inoue1, Miki Ueda1, Rika Osawa1, Satoshi Kanoh1, David J. Hartshorne3, Suechika Suzuki2 and Shugo Watabe4,{dagger}

1 Graduate School of Bioresources, Mie University, Tsu, Mie 514-8507, Japan
2 Faculty of Science, Kanagawa University, Hiratsuka, Kanagawa 259-1293, Japan
3 Muscle Biology Group, University of Arizona, Tucson, AZ 85721, USA
4 Graduate School of Agricultural and Life Sciences, The University of Tokyo, Bunkyo, Tokyo 113-8567, Japan

{dagger} Author for correspondence (e-mail: awatabe{at}mail.ecc.u-tokyo.ac.jp)

Accepted 18 September 2007

Molluscan smooth muscle can maintain tension over extended periods with little energy expenditure, a process termed catch. Catch is thought to be regulated by phosphorylation of a thick filament protein, twitchin, and involves two phosphorylation sites, D1 and D2, close to the N and C termini, respectively. This study was initiated to investigate the role of the D2 site and its phosphorylation in the catch mechanism. A peptide was constructed containing the D2 site and flanking immunoglobulin (Ig) motifs. It was shown that the dephosphorylated peptide, but not the phosphorylated form, bound to both actin and myosin. The binding site on actin was within the sequence L10 to P29. This region also binds to loop 2 of the myosin head. The dephosphorylated peptide linked myosin and F-actin and formed a trimeric complex. Electron microscopy revealed that twitchin is distributed on the surface of the thick filament with an axial periodicity of 36.25 nm and it is suggested that the D2 site aligns with the myosin heads. It is proposed that the complex formed with the dephosphorylated D2 site of twitchin, F-actin and myosin represents a component of the mechanical linkage in catch.

Key words: catch contraction, twitchin, ABRM, myosin, actin






© The Company of Biologists Ltd 2007