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J. Cell Biol., Volume 140, Number 5, March 9, 1998 1113-1124

MKBP, a Novel Member of the Small Heat Shock Protein Family, Binds and Activates the Myotonic Dystrophy Protein Kinase

Atsushi Suzuki,* Yuki Sugiyama,* Yukiko Hayashi, Nobuo Nyu-i,*§ Michihiko Yoshida,*Dagger Ikuya Nonaka, Sho-ichi Ishiura,par Kiichi Arahata, and Shigeo Ohno*

* Department of Molecular Biology, Dagger  The First and § Second Departments of Internal Medicine, Yokohama City University School of Medicine, Kanazawa-ku, Yokohama 236, Japan; par  Institute of Molecular and Cellular Biosciences, the University of Tokyo, Tokyo 113, Japan; and  National Institute of Neuroscience, National Center of Neurology and Psychiatry, Kodaira, Tokyo 187, Japan

Muscle cells are frequently subjected to severe conditions caused by heat, oxidative, and mechanical stresses. The small heat shock proteins (sHSPs) such as alpha B-crystallin and HSP27, which are highly expressed in muscle cells, have been suggested to play roles in maintaining myofibrillar integrity against such stresses. Here, we identified a novel member of the sHSP family that associates specifically with myotonic dystrophy protein kinase (DMPK). This DMPK-binding protein, MKBP, shows a unique nature compared with other known sHSPs: (a) In muscle cytosol, MKBP exists as an oligomeric complex separate from the complex formed by alpha B-crystallin and HSP27. (b) The expression of MKBP is not induced by heat shock, although it shows the characteristic early response of redistribution to the insoluble fraction like other sHSPs. Immunohistochemical analysis of skeletal muscle cells shows that MKBP localizes to the cross sections of individual myofibrils at the Z-membrane as well as the neuromuscular junction, where DMPK has been suggested to be concentrated. In vitro, MKBP enhances the kinase activity of DMPK and protects it from heat-induced inactivation. These results suggest that MKBP constitutes a novel stress-responsive system independent of other known sHSPs in muscle cells and that DMPK may be involved in this system by being activated by MKBP. Importantly, since the amount of MKBP protein, but not that of other sHSP family member proteins, is selectively upregulated in skeletal muscle from DM patients, an interaction between DMPK and MKBP may be involved in the pathogenesis of DM.


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