SMYD1-mediated Mono-Methylation of Lysine K35 of the sarcomeric Myosin Heavy Chain (MHC) is fundamental for thick filament assembly in zebrafish and human iPSC-derived cardiomyocytes

The SMYD family is a unique class of lysine methyltransferases (KMTases) known to methylate histones but also non-histone proteins. Among the five SMYD family members (1-5), SMYD1 was identified as a heart- and skeletal muscle-specific KMTase, which, together with Unc45b and Hsp90a, interacts with Myosin thereby regulating thick filament assembly. However, the process by which SMYD1 orchestrates Myosin assembly is largely unknown. Here, we found that SMYD1 physically interacts with Myosin heavy chain (Myh) at its N-terminus and that the Myh N-terminus specifically gets mono-methylated by SMYD1 at lysine 35 (K35). Accordingly, methylated Myh is properly integrated into functional sarcomeres, whereas unmethylated Myh molecules in Smyd1-deficient zebrafish are efficiently degraded by the Ubiquitin Proteasome System (UPS) leading to defective thick filament assembly. Although the inhibition of the UPS by MG132 is able to reconstitute Myosin levels in Smyd1-deficient zebrafish embryos, thick filament assembly is still blocked due to the lack of K35 Myh mono-methylation. Similar to the situation in zebrafish striated muscle cells, SMYD1-mediated MYH methylation is also critical for thick filament assembly in human cardiomyocytes, indicating cross-species conservation of this fundamental mechanism of Myosin methylation, which has been first described about 40 years ago. Further investigations will now be essential to explore the therapeutic potential of targeting this pathway in cardiomyopathies and skeletal muscle disorders.