Background: 

Protein lysine methylation is one of the most widespread posttranslational modifications in the nuclei of eukaryotic cells. Methylated lysines on histones and non-histone proteins promote the formation of protein complexes that control gene expression, DNA replication and repair. In the cytoplasm, however, the role of lysine methylation in protein complex formation is not well established.

Results: 

We report that the cytoplasmic protein chaperone Hsp90 is methylated by the lysine methyltransferase Smyd2, which localizes predominantly to the cytoplasm and is highly abundant in skeletal muscle and myocardium. Methylation of Hsp90 is also strongest in striated muscle. We identify a specific site for Hsp90 lysine methylation at position K616 (dimerization domain) and show that methylation of this site critically depends on Smyd2. In skeletal myocytes, Hsp90 methylation contributes to the formation of a protein complex containing Smyd2, Hsp90 and the giant sarcomeric protein titin, specifically the N2A domain contained within the elastic spring segment of titin. Knockdown of Smyd2 in zebrafish results in the loss of Hsp90 methylation and impaired stability of I-band titin, but not M-band titin. Smyd2 deficiency causes disorganization of the sarcomeric Z-disk and I-band regions (but not the A-band/M-band region) and skeletal muscle dysfunction.

Conclusions: 

Our data reveal a novel regulatory cytoplasmic protein network involving the lysine methyltransferase Smyd2, the master chaperone Hsp90, and the elastic titin region. This network employs lysine methylation for the maintenance and function of skeletal muscle in vivo.