Small Leucine Zipper Protein (sLZIP) Negatively Regulates Skeletal Muscle Differentiation via Interaction with α-Actinin-4*

Background: Cooperation between transcription factors and myogenic regulatory factors is important for skeletal muscle differentiation. Results: sLZIP inhibits expression of muscle-specific genes during myogenesis via disruption of an association between -actinin-4 and MEF2. Conclusion: A novel myogenesis regulatory mechanism of sLZIP is characterized. Significance: sLZIP can be used as a therapeutic target for treatment of muscle diseases. The small leucine zipper protein (sLZIP) plays a role in transcriptional regulation in various types of cells. However, the role of sLZIP in myogenesis is unknown. We identified -actinin-4 (ACTN4) as a sLZIP-binding protein. ACTN4 functions as a transcriptional regulator of myocyte enhancer factor (MEF)2, which plays a critical role in expression of muscle-specific genes during skeletal muscle differentiation. We found that ACTN4 translocates to the nucleus, induces myogenic gene expression, and promotes myotube formation during myogenesis. The myogenic process is controlled by an association between myogenic factors and MEF2 transcription factors. ACTN4 increased expression of muscle-specific proteins via interaction with MEF2. However, sLZIP decreased myogenic gene expression and myotube formation during myogenesis via disruption of the association between ACTN4 and MEF2. ACTN4 increased the promoter activities of myogenic genes, whereas sLZIP abrogated the effect of ACTN4 on transcriptional activation of myogenic genes in myoblasts. The C terminus of sLZIP is required for interaction with the C terminus of ACTN4, based on deletion mutant analysis, and sLZIP plays a role in regulation of MEF2 transactivation via interaction with ACTN4. Our results indicate that sLZIP negatively regulates skeletal muscle differentiation via interaction with ACTN4 and that sLZIP can be used as a therapeutic target molecule for treatment of muscle hypertrophy and associated diseases.