SUMMARY Histone acetylation plays critical roles in chromatin remodeling, DNA repair, and epigenetic regulation of gene expression, but the underlying mechanisms are unclear. Proteasomes usually catalyze ATP- and polyubiquitin-dependent proteolysis. Here we show that the proteasomes containing the activator PA200 catalyze the polyubiquitin-independent degradation of histones. Most proteasomes in mammalian testes (“spermatoproteasomes”) contain a spermatid/sperm-specific α-subunit α4s/PSMA8 and/or the catalytic β-subunits of immunoproteasomes in addition to PA200. Deletion of PA200 in mice abolishes acetylation-dependent degradation of somatic core histones during DNA double-strand breaks, and delays core histone disappearance in elongated spermatids. Purified PA200 greatly promotes ATP-independent proteasomal degradation of the acetylated core histones, but not polyubiquitinated proteins. Furthermore, acetylation on histones is required for their binding to the bromodomain-like regions in PA200 and its yeast ortholog, Blm10. Thus, PA200/Blm10 specifically targets the core histones for acetylation-mediated degradation by proteasomes, providing mechanisms by which acetylation regulates histone degradation, DNA repair, and spermatogenesis.

Targeted gene disruption studies have established that the c-Jun NH2-terminal kinase (JNK) is required for the stress-induced release of mitochondrial cytochrome c and apoptosis, and that the Bax subfamily of Bcl-2-related proteins is essential for JNK-dependent apoptosis. However, the mechanism by which JNK regulates Bax has remained unsolved. Here we demonstrate that activated JNK promotes Bax translocation to mitochondria through phosphorylation of 14-3-3, a cytoplasmic anchor of Bax. Phosphorylation of 14-3-3 led to dissociation of Bax from this protein. Expression of phosphorylation-defective mutants of 14-3-3 blocked JNK-induced Bax translocation to mitochondria, cytochrome c release and apoptosis. Collectively, these results have revealed a key mechanism of Bax regulation in stress-induced apoptosis

Voltage-gated calcium channels play a central role in regulating the electrical and biochemical properties of neurons and muscle cells. One of the ways in which calcium channels regulate long-lasting neuronal properties is by activating signaling pathways that control gene expression, but the mechanisms that link calcium channels to the nucleus are not well understood. We report that a C-terminal fragment of Ca(V)1.2, an L-type voltage-gated calcium channel (LTC), translocates to the nucleus and regulates transcription. We show that this calcium channel associated transcription regulator (CCAT) binds to a nuclear protein, associates with an endogenous promoter, and regulates the expression of a wide variety of endogenous genes important for neuronal signaling and excitability. The nuclear localization of CCAT is regulated both developmentally and by changes in intracellular calcium. These findings provide evidence that voltage-gated calcium channels can directly activate transcription and suggest a mechanism linking voltage-gated channels to the function and differentiation of excitable cells.

Life and death decisions are made by integrating a variety of apoptotic and survival signals in mammalian cells. Therefore, there is likely to be a common mechanism that integrates multiple signals adjudicating between the alternatives. In this study, we propose that 14-3-3 represents such an integration point. Several proapoptotic proteins commonly become associated with 14-3-3 upon phosphorylation by survival-mediating kinases such as Akt. We reported previously that cellular stresses induce c-Jun NH2-terminal kinase (JNK)–mediated 14-3-3ζ phosphorylation at Ser184 (Tsuruta, F., J. Sunayama, Y. Mori, S. Hattori, S. Shimizu, Y. Tsujimoto, K. Yoshioka, N. Masuyama, and Y. Gotoh. 2004. EMBO J. 23:1889–1899). Here, we show that phosphorylation of 14-3-3 by JNK releases the proapoptotic proteins Bad and FOXO3a from 14-3-3 and antagonizes the effects of Akt signaling. As a result of dissociation, Bad is dephosphorylated and translocates to the mitochondria, where it associates with Bcl-2/Bcl-xL. Because Bad and FOXO3a share the 14-3-3–binding motif with other proapoptotic proteins, we propose that this JNK-mediated phosphorylation of 14-3-3 regulates these proapoptotic proteins in concert and makes cells more susceptible to apoptotic signals.

Bax, a proapoptotic member of the Bcl-2 family, localizes largely in the cytoplasm but redistributes to mitochondria in response to apoptotic stimuli, where it induces cytochrome c release. In this study, we show that the phosphatidylinositol 3-OH kinase (PI3K)-Akt pathway plays an important role in the regulation of Bax subcellular localization. We found that LY294002, a PI3K inhibitor, blocked the effects of serum to prevent Bax translocation to mitochondria and that expression of an active form of PI3K suppressed staurosporine-induced Bax translocation, suggesting that PI3K activity is essential for retaining Bax in the cytoplasm. In contrast, both U0126, a MEK inhibitor, and active MEK had little effect on Bax localization. In respect to downstream effectors of PI3K, we found that expression of active Akt, but not serum and glucocorticoid-induced protein kinase (SGK), suppressed staurosporine-induced translocation of Bax, whereas dominant negative Akt moderately promoted Bax translocation. Expression of Akt did not alter the levels of Bax, Bcl-2, Bcl-X L , or phosphorylated JNK under the conditions used, suggesting that there were alternative mechanisms for Akt in the suppression of Bax translocation. Collectively, these results suggest that the PI3K-Akt pathway inhibits Bax translocation from cytoplasm to mitochondria and have revealed a novel mechanism by which the PI3K-Akt pathway promotes survival.