The class II deacetylase histone deacetylase 4 (HDAC4) negatively regulates the transcription factor MEF2. HDAC4 is believed to repress MEF2 transcriptional activity by binding to MEF2 and catalyzing local histone deacetylation. Here we report that HDAC4 also controls MEF2 by a novel SUMO E3 ligase activity. We show that HDAC4 interacts with the SUMO E2 conjugating enzyme Ubc9 and is itself sumoylated. The overexpression of HDAC4 leads to prominent MEF2 sumoylation in vivo, whereas recombinant HDAC4 stimulates MEF2 sumoylation in a reconstituted system in vitro. Importantly, HDAC4 promotes sumoylation on a lysine residue that is also subject to acetylation by a MEF2 coactivator, the acetyltransferase CBP, suggesting a possible interplay between acetylation and sumoylation in regulating MEF2 activity. Indeed, MEF2 acetylation is correlated with MEF2 activation and dynamically induced upon muscle cell differentiation, while sumoylation inhibits MEF2 transcriptional activity. Unexpectedly, we found that HDAC4 does not function as a MEF2 deacetylase. Instead, the NAD ؉ -dependent deacetylase SIRT1 can potently induce MEF2 deacetylation. Our studies reveal a novel regulation of MEF2 transcriptional activity by two distinct classes of deacetylases that affect MEF2 sumoylation and acetylation.Precise temporal and spatial gene expression is critical for the execution of the differentiation program and other important biological processes. This tight regulation of gene expression is achieved by the interplay of transcriptional activation and repression controlled by transcription factors that recruit specific cofactor complexes capable of modifying histones and the local chromatin structure (reviewed in references 12 and 29). Among the cofactors that are involved in transcriptional repression, members of the histone deacetylase (HDAC) family are the most well characterized. Numerous studies have reported the recruitment of HDAC members by transcription factors to repress gene expression. Based on the long-established correlation between histone acetylation and gene transcription, it is generally thought that HDAC members repress gene transcription by catalyzing local histone deacetylation (reviewed in reference 21). However, whether this mechanism is universally applicable to HDAC-mediated transcriptional repression is not known.Among HDAC family members involved in transcriptional regulation, HDAC4 and HDAC5 are closely related and belong to a subfamily that also includes HDAC7 and HDAC9 (reviewed in reference 37). This subfamily of HDACs share several unique properties. First, they all contain the N-terminal noncatalytic MITR (MEF2-interacting transcription repressor) homology domain (37). Second, they are all regulated by phosphorylation-dependent cytoplasmic-nuclear trafficking (18,23,45). Third, and most importantly, they are all critical regulators of MEF2, a family of transcription factors important in muscle cell differentiation and apoptosis (reviewed in reference 24). In skeletal muscle cells, MEF2 members coll...

Summary Gene expression requires proper messenger (m) RNA export and translation. However, the functional links between these consecutive steps have not been fully defined. Gle1 is an essential, conserved mRNA export factor whose export function is dependent on the small molecule inositol hexakisphosphate (IP6). Here we show that both Gle1 and IP6 are required for efficient translation termination in Saccharomyces cerevisiae, and Gle1 interacts with termination factors. In addition, Gle1 has a conserved physical association with the initiation factor eIF3, and gle1 mutants display genetic interactions with the eIF3 mutant nip1-1. Strikingly, gle1 mutants have defects in initiation, whereas strains lacking IP6 do not. We propose that Gle1 functions together with IP6 and the DEAD-box protein Dbp5 to regulate termination. However, Gle1 also independently mediates initiation. Thus, Gle1 is uniquely positioned to coordinate the mRNA export and translation mechanisms. These results directly impact models for perturbation of Gle1 function in pathophysiology.

Histone deacetylase 4 (HDAC4) undergoes signal-dependent shuttling between the cytoplasm and nucleus, which is regulated in part by calcium/calmodulin-dependent kinase (CaMK)-mediated phosphorylation. Here, we report that HDAC4 intracellular trafficking is important in regulating neuronal cell death. HDAC4 is normally localized to the cytoplasm in brain tissue and cultured cerebellar granule neurons (CGNs). However, in response to low-potassium or excitotoxic glutamate conditions that induce neuronal cell death, HDAC4 rapidly translocates into the nucleus of cultured CGNs. Treatment with the neuronal survival factor BDNF suppresses HDAC4 nuclear translocation, whereas a proapoptotic CaMK inhibitor stimulates HDAC4 nuclear accumulation. Moreover, ectopic expression of nuclear-localized HDAC4 promotes neuronal apoptosis and represses the transcriptional activities of myocyte enhancer factor 2 and cAMP response element-binding protein, survival factors in neurons. In contrast, inactivation of HDAC4 by small interfering RNA or HDAC inhibitors suppresses neuronal cell death. Finally, an increase of nuclear HDAC4 in granule neurons is also observed in weaver mice, which harbor a mutation that promotes CGN apoptosis. Our data identify HDAC4 and its intracellular trafficking as key effectors of multiple pathways that regulate neuronal cell death.

In C2C12 myoblasts, endogenous histone deacetylase HDAC4 shuttles between cytoplasmic and nuclear compartments, supporting the hypothesis that its subcellular localization is dynamically regulated. However, upon differentiation, this dynamic equilibrium is disturbed and we find that HDAC4 accumulates in the nuclei of myotubes, suggesting a positive role of nuclear HDAC4 in muscle differentiation. Consistent with the notion of regulation of HDAC4 intracellular trafficking, we reveal that HDAC4 contains a modular structure consisting of a C-terminal autonomous nuclear export domain, which, in conjunction with an internal regulatory domain responsive to calcium/calmodulin-dependent protein kinase IV (CaMKIV), determines its subcellular localization. CaMKIV phosphorylates HDAC4 in vitro and promotes its nuclear-cytoplasmic shuttling in vivo. However, although 14-3-3 binding of HDAC4 has been proposed to be important for its cytoplasmic retention, we find this interaction to be independent of CaMKIV. Rather, the HDAC4⅐14-3-3 complex exists in the nucleus and is required to confer CaMKIV responsiveness. Our results suggest that the subcellular localization of HDAC4 is regulated by sequential phosphorylation events. The first event is catalyzed by a yet to be identified protein kinase that promotes 14-3-3 binding, and the second event, involving protein kinases such as CaMKIV, leads to efficient nuclear export of the HDAC4⅐14-3-3 complex.Accumulating evidence indicates that active transcriptional repression is an important component of many physiological events regulated at the level of gene expression, including muscle differentiation (1). The repression of transcription is manifest at the level of chromatin structure where histone deacetylases (HDACs) 1 are recruited to deacetylate histones and create a repressive chromatin structure (reviewed in Ref.2). Of the ten human HDACs identified so far (3), 2 HDAC4 and its closely related family member HDAC5 have been specifically implicated in regulating muscle differentiation ((1) and see below).The functional link between HDAC4/5 and muscle differentiation was first uncovered by the cloning of MITR, a transcriptional repressor identified as an interactive partner for myocyte enhancer factor 2 (MEF-2) transcription factor family members, which are important for muscle differentiation (4). MITR shows extensive homology to the non-catalytic N terminus of HDAC4 and -5 (4). Indeed both HDAC4 and HDAC5 interact with MEF-2. It was reported that overexpression of HDAC4 or HDAC5 represses MEF-2 transcriptional activity (5) and suppresses C2C12 myoblast differentiation (1). It was also found that the HDAC4/5⅐MEF-2 interaction and the effect of this complex on muscle differentiation could be reversed by a constitutively active form of a calcium/calmodulin-dependent protein kinase (CaMK) (6). However, the mechanism by which CaMK regulates HDAC4 and HDAC5 is not entirely clear.When ectopically expressed, HDAC4 can be found in either the nucleus or cytoplasm whereas the closely relat...