Muscle develops a specific form of small heat shock protein complex composed of MKBP/HSPB2 and HSPB3 during myogenic differentiation

Previously, we identified a new mammalian sHSP, MKBP, as a myotonic dystrop hy protein kinase-binding protein, and suggested its important role in musc le maintenance (Suzuki, A., Sugiyama, Y., Hayashi, Y., Nyu-i, N., Yoshida, M., Nonaka, I., Ishiura, S., Arahata, K., and Ohno, S. (1998) J. Cell Biol. 140, 1113-1124). In this paper, we develop the former work by performing e xtensive characterization of five of the six sHSPs so far identified, that is, HSP27, alpha B-crystallin, p20, MKBP/HSPB2, and HSPB3, omitting lens-sp ecific alpha A-crystallin. Tissue distribution analysis revealed that altho ugh each sHSP shows differential constitutive expression in restricted tiss ues, tissues that express all five sHSPs are only muscle-related tissues. E specially, the expressions of HSPB3, identified for the first time as a 17- kDa protein in this paper, and MKBP/HSPB2 are distinctly specific to muscle s. Moreover, these sHSPs form an oligomeric complex with an apparent molecu lar mass of 150 kDa that is completely independent of the oligomers formed by HSP27, alpha B-crystallin, and p20. The expressions of MKBP/HSPB2 and HS PB3 are induced during muscle differentiation under the control of MyoD, su ggesting that the sHSP oligomer comprising MKBP/HSPB2 and HSPB3 represents an additional system closely related to muscle function. The functional div ergence among sHSPs in different oligomers is also demonstrated in several ways: 1) an interaction with myotonic dystrophy protein kinase, which has b een suggested to be important for the maintenance of myofibril integrity, w as observed only for MKBP/HSPB2; 2) a myotube-specific association with act in bundles was observed for HSP27 and alpha B-crystallin, but not for MKBP/ HSPB2; and 3) sHSPs whose mRNAs are induced by heat shock are alpha B-cryst allin and HSP27. Taken together, the results suggest that muscle cells deve lop two kinds of stress response systems composed of diverged sHSP members, and that these systems work independently in muscle maintenance and differ entiation.