The molecular basis of thymocyte negative selection, a crucial mechanism in establishing central tolerance, is not yet resolved. Histone deacetylases (HDACs) have emerged as key transcriptional regulators in several major developmental programs. Recently, we showed that the class IIa member, HDAC7, regulates negative selection by repressing expression of Nur77, an orphan nuclear receptor involved in antigen-induced apoptosis of thymocytes. Engagement of the T cell receptor (TCR) alleviates this repression through phosphorylation-dependent nuclear exclusion of HDAC7. However, the identity of the TCR-activated kinase that phosphorylates and inactivates HDAC7 was still unknown. Here, we demonstrate that TCR-induced nuclear export of HDAC7 and Nur77 expression is mediated by activation of protein kinase D (PKD). Indeed, active PKD stimulates HDAC7 nuclear export and Nur77 expression. In contrast, inhibition of PKD prevents TCR-mediated nuclear exclusion of HDAC7 and associated Nur77 activation. Furthermore, we show that HDAC7 is an interaction partner and a substrate for PKD. We identify four serine residues in the NH2 terminus of HDAC7 as targets for PKD. More importantly, a mutant of HDAC7 specifically deficient in phosphorylation by PKD, inhibits TCR-mediated apoptosis of T cell hybridomas. These findings indicate that PKD is likely to play a key role in the signaling pathways controlling negative selection.

Class IIa histone deacetylases (HDACs) are found both in the cytoplasm and in the nucleus where they repress genes involved in several major developmental programs. In response to specific signals, the repressive activity of class IIa HDACs is neutralized through their phosphorylation on multiple N-terminal serine residues and 14-3-3-mediated nuclear exclusion. Here, we demonstrate that class IIa HDACs are subjected to signal-independent nuclear export that relies on their constitutive phosphorylation. We identify EMK and C-TAK1, two members of the microtubule affinity-regulating kinase (MARK)/Par-1 family, as regulators of this process. We further show that EMK and C-TAK1 phosphorylate class IIa HDACs on one of their multiple 14-3-3 binding sites and alter their subcellular localization and repressive function. Using HDAC7 as a paradigm, we extend these findings by demonstrating that signal-independent phosphorylation of the most N-terminal serine residue by the MARK/Par-1 kinases, i.e., Ser 155 , is a prerequisite for the phosphorylation of the nearby 14-3-3 site, Ser 181 . We propose that this multisite hierarchical phosphorylation by a variety of kinases allows for sophisticated regulation of class IIa HDACs function.