The deacetylase HDAC6 is an essential component of stress granules and plays a critical role in the cellular response to stress

Abstract

The reversible acetylation of histones has a critical role in transcriptional regulation. Likewise reversible acetylation of non-histones proteins is also important for other cellular processes. Acetylation and deacetylation of histones and other proteins are catalyzed by opposing histone acetyl transferases (HATs) and deacetylases (HDACs) respectively. Among three classes of histone deacetylases, HDAC6 is a very unique class II HDAC enzyme which possesses two independent deacetylase domains and a Zn-UBP ubiquitin binding domain at the C-terminus. HDAC6 has been shown to interact with nuclear proteins as well as cytoplasmic proteins such as tubulin and HSP90. However, the physiological function of HDAC6 is not fully understood yet. Therefore, to further define the cellular function of HDAC6, an identification of novel interacting proteins has been undertaken. The first section of this thesis describes the identification of one novel HDAC6 interacting protein and the role of HDAC6 in stress granule (SG) formation in response to environmental stress. First of all, we identified new HDAC6 interacting proteins using proteomic affinity trap approach. Here, we focused that HDAC6 interacts and colocalizes with a previously identified stress granule component, G3BP (RasGAP associated endoribonuclease) in vitro and in vivo. We first discovered that HDAC6 is a stable and critical component of stress granules. Further experimental data suggested that HDAC6 can regulate the assembly of SGs via recruiting SG components to the microtubule system. Because of this HDAC6 may have an impact on various processes involoving RNA metabolism and we provide initial evidence that the miRNA pathway is indeed influenced by HDAC6 function. The second section of this thesis examines the role of HDAC6 in response to various stresses. The involvement of HDAC6, a multi-functional cytoplasmic deacetylase, in processes such as the clearance of cytotoxic aggregated misfolded proteins and the deacetylation of HSP90 chaperone, has prompted us to investigate a role for HDAC6 in cellular protection under stress condition. In hypoxia, HDAC6 regulated stability of HIF- 1α by controlling its deacetylation. Indeed, loss of HDAC6 rendered cells more sensitive to programmed cell death. Moreover, depletion of HDAC6 affected the recovery of cells from stress as well as the direct stress response, suggesting a significant role of HDAC6 as a cellular regulator of the stress response. In addition, a number of other putative HDAC6 interactors are presented, which were identified in the initial mass spectrometry screens. Several of these proteins encode cytoplasmic factors that have a role in RNA metabolism, protein translation or in cytoskeletal regulation. Therefore, it appears likely that at least some of these may turn out to be relevant partners contributing to HDAC6 function.