m-Bop, a Repressor Protein Essential for Cardiogenesis, Interacts with skNAC, a Heart- and Muscle-specific Transcription Factor

Abstract

The m-Bop protein encoded by the mouse Bop gene is strongly expressed in heart and skeletal muscle, and recent studies with Bop knockout mice have demonstrated that m-Bop is essential for cardiogenesis in vivo and can act as a HDAC-dependent repressor in vitro. In the present studies, m-Bop was observed to interact with skNAC, a reported transcriptional activator specific to heart and skeletal muscle. The amino-terminal S region of the split S-ET domain of m-Bop as well as the MYND domain were required for interaction with skNAC in both the two-hybrid system and in coimmunoprecipitation experiments from cultured mammalian cells. As shown previously for interaction of the MYND domain-containing transcriptional corepressor, BS69, with several viral and cellular oncoproteins, a PXLXP motif in skNAC was required for interaction with mBop. Similar kinetics of induction and localization of m-Bop and skNAC during the induction of myogenesis in cultured C2C12 cells suggests a possible associated role for these proteins during this process.The control of skeletal and cardiac maturation is ordered in a complex cascade of transcriptional activation and repression. The MyoD and MEF2 families of transcription factors dictate important developmental events that result in the formation of mature skeletal muscle. The MyoD family is comprised of the basic helix-loop-helix proteins MyoD, Myf5, MRF4, and myogenin. These myogenic regulators form heterodimers with the ubiquitous basic helix-loop-helix E proteins that subsequently activate key elements needed for the myogenic program (1). The MEF2 family is defined by the MADS domain and consists of MEF2A, MEF2B, MEF2C, and MEF2D. Members of the MEF2 family are expressed in tissues besides skeletal muscle including cardiac tissue, neurons, and T cells (2). Together, the MyoD and MEF2 families cooperate directly and indirectly to transduce the requisite signals for proper skeletal muscle formation.The discovery of chamber-specific transcription factors, such as the Hand1 and Hand2 proteins, has revealed the complex nature of cardiac transcriptional regulation (3). The list of genes that correspond to specific cardiac defects continues to grow, whereas the molecular nature of these defects remains largely elusive (4). To further understand the molecular underpinnings of cardiac development, it is critical to identify the relationships of transcriptional regulators shown to be important in cardiac morphogenesis.The Bop gene encodes distinct proteins expressed in skeletal and cardiac muscle, as well as in cytotoxic T lymphocytes. The Bop proteins found in skeletal muscle (m-Bop) and cytotoxic T lymphocytes (t-Bop) are identical over 90% of their primary amino acid sequence, differing only at their extreme amino terminus (5). Bop contains the evolutionarily conserved MYND and SET domains found in transcriptional regulators linked to development, chromatin stability, and cancer. The MYND domains in the transcriptional regulators ETO (MTG8) and BS69 function as protein-protein interaction d...

The Nuclear Receptor Co-repressor (N-CoR) Utilizes Repression Domains I and III for Interaction and Co-repression with ETO

“…BS69 and m-Bop have been shown to interact with a conserved binding motif, PXLXP, in cellular and oncoviral proteins, an interaction that is dependent on the MYND domain (38,(43)(44)(45). Consistent with these observations, ETO NHR3/4 binds a motif with a PPLXP pattern in N-CoR (Fig.…”

“…The MYND-type zinc fingers resident in NHR4 of ETO have also been identified in other co-repressor proteins, notably human BS69, the skeletal and heart muscle specific m-Bop (mouse CD8b opposite) protein in mice, the Caenorhabditis elegans proteins BRA-1 and BRA-2, and the Myc-related cellular protein MGA (38,43). BS69 and m-Bop have been shown to interact with a conserved binding motif, PXLXP, in cellular and oncoviral proteins, an interaction that is dependent on the MYND domain (38,(43)(44)(45).…”

“…5) (38). An examination of the N-CoR RIII amino acid sequence suggested that it too contained this conserved interaction motif (Fig.…”

Histone Methyltransferase SETD3 Regulates Muscle Differentiation

“…Collectively, these findings demonstrate that SETD3 exerts specific HMTase activity in the mono-and dimethylation of H3K4 and H3K36 under both in vitro and in vivo conditions. SETD3 Induces Transcriptional Activation-It was noted previously that histone H3K4 HMTase SMYD1 functions as a transcription activator, playing an essential role in myogenesis and cardiogenesis in different species (6,(25)(26)(27). To determine whether or not methylation of H3K4 and H3K36 by SETD3 can be attributed to a general transcription activation effect similar to that of SMYD1, we conducted a transient transfection assay using the CMX-GAL4-SV40 reporter system.…”

The E3 SUMO ligase Nse2 regulates sumoylation and nuclear-to-cytoplasmic translocation of skNAC-Smyd1 in myogenesis

“…Finally, a function for skNAC in transcription regulation has also been suggested by Sims and colleagues (Sims et al, 2002); the authors showed for the first time that skNAC binds to the m-Bop/Smyd1 protein [note, muscle (m)-Bop is the isoform of the the Smyd1 gene that is expressed in muscles], a SET-domaincontaining histone methyltransferase that also contains a MYND domain known to be involved in the recruitment of histone deacetylases (HDACs). Interestingly, the skNAC-Smyd1 complex localizes to the nucleus in myoblasts at an early differentiation stage and then translocates to the cytosol at later stages of differentiation, where at least Smyd1 is associated with sarcomeric structures (Just et al, 2011;Li et al, 2011), suggesting that, besides regulating transcription, the proteins might also exert a function in the cytoplasm at later stages of myogenesis.…”