Molecular chaperones Hsp90a1 and Unc45b are required for sarcomere assembly in skeletal muscles of zebrafish embryos

Abstract

Heat shock protein 90α (Hsp90α) is a member of the chaperone family involved in protein folding and assembly. The role of Hsp90α in the developmental process, however, remains unclear. Here we report that Hsp90α1 is specifically expressed in skeletal muscles of zebrafish embryos. Knockdown of Hsp90α1 expression resulted in paralyzed zebrafish embryos with poorly organized myofibrils in skeletal muscles. Biochemical analyses revealed that knockdown of Hsp90α1 expression resulted in myosin degradation and up-regulation of unc45b gene expression in zebrafish embryos. Unc-45b is a myosin chaperone and a Hsp90α co-chaperone that plays a key role in muscle development. To determine whether upregulation of Unc45b expression is detrimental to sarcomere assembly, we directly overexpressed Unc45b in skeletal muscles of zebrafish embryos via DNA injection. Embryos overexpressing Unc45b exhibited severely disorganized myosin thick filaments. Disruption of thick filament organization by Unc45b overexpression depended on its C-terminal UCS domain required for interaction with myosin. Deletion of the C-terminal UCS domain abolished the disruptive activity of Unc45b in myosin thick filament organization. In contrast, deletion of the N-terminal TPR domain required for binding with Hsp90α had no effect. Collectively, these studies indicate that Hsp90α1 and Unc45b play vital roles in sarcomere assembly and their expression levels must be precisely regulated to ensure normal myofibril organization. Supported by NIH training grant (#T32 AR07592) and a research grant MB-8713-08 from BARD.