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...

The exocyst complex plays multiple roles via tethering secretory or recycling vesicles to the plasma membrane. Previous studies have demonstrated that the exocyst contains eight components, which possibly have some redundant but distinct functions. It is therefore interesting to investigate the biological function of each component. Here, we found that Sec3, one component of exocyst complex, is involved in Drosophila egg chamber development. Loss of sec3 results in egg chamber fusion through the abolishment of cell differentiation. In addition, loss of sec3 increases cell numbers but decreases cell size. These defects phenocopy Notch pathway inactivation. In line with this, loss of sec3 indeed leads to Notch protein accumulation, suggesting that the loss of Sec3 inhibits the delivery of Notch onto the plasma membrane and accumulates inactive Notch in the cytoplasm. Loss of sec3 also leads to the ectopic expression of two Notch pathway target genes, Cut and FasciclinIII, which should normally be downregulated by Notch. Altogether, our study revealed that Sec3 governs egg chamber development through the regulation of Notch, and provides fresh insights into the regulation of oogenesis.

Abstract. Cell migration plays an important role in many physiological and pathological processes. Understanding of the mechanisms of cell migration may lead us to develop novel therapeutic strategies for controlling human disease. Border cell migration in the ovary of Drosophila melanogaster has emerged as an excellent model for studying cell migration in vivo. In a previous study, we have found that the gene sec3 that encodes a component of Drosophila exocyst is required for border cell migration. In the following study, we found that high levels of Sec3-GFP are seen in the leading edge of migratory border cells, and sec3 functions together with other genes encoding exocyst components. Moreover, knockdown of exocyst components induces aberrant wing development. The results implicates that exocyst components play important roles in Dosophila border cell migration and wing development, and function as a complex as they have been found in yeast.