Smyd1b_tv1, a key regulator of sarcomere assembly, requires Ser225 for its sarcomeric localization

Abstract

Smyd1b is a member of the Smyd family that contains the highly conserved SET and MYND domains. smyd1b encodes two alternatively spliced isoforms, smyd1b_tv1 and smyd1b_tv2, that are expressed in skeletal and cardiac muscles. Smyd1b_tv1 differs from Smyd1b_tv2 by a 13 amino acid insertion in the SET domain. Smyd1b plays a key role in sarcomere assembly during myofibrillogenesis. Knockdown of smyd1b expression resulted in paralyzed zebrafish larvae with defective sarcomere assembly in skeletal muscles. To better understand Smyd1b function in myofibrillogenesis, we analyzed the subcellular localization of Smyd1b_tv1 and Smyd1b_tv2 in zebrafish embryos. Smyd1b_tv1 and Smyd1b_tv2 exhibited different patterns of subcellular localization during muscle development. Smyd1b_tv1 and Smyd1b_tv2 were primarily localized in the cytosol of myoblasts and myotubes at early stage zebrafish embryos. However, Smyd1b_tv1 and Smyd1b_tv2 exhibited distinct subcellular localization in differentiated myofibers. Smyd1b_tv1 showed a strong sarcomeric localization compared with Smyd1b_tv2. The sarcomeric localization depended on Ser225 located within the Smyd1b-tv1-specific 13 aa insertion. S225A substitution abolished its sarcomeric localization. However, S225T substitution retained the sarcomeric localization of Smyd1b-tv1. Together, these data indicate that Smyd1b-tv1 may play a direct role in sarcomere assembly, and post-translational modification by phosphorylation may modulate Smyd1b_tv1 sarcomeric localization and function. Supported by a research grant MB-8716-08 from BARD.