Browsing by Subject "HAT"

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    Functions of the Unique N-terminus of a GCN5 Histone Acetylase in Toxoplasma gondii
    (2007-05-18T13:14:16Z) Bhatti, Micah M.; Sullivan, William J., Jr.; Chan, Edward M.; Queener, Sherry F.; Safa, Ahmad R.; Sinai, Anthony P.; Vasko, Michael, R.
    GCN5 is a histone acetyltransferase (HAT) that remodels chromatin by acetylating lysine residues of histones. The GCN5 HAT identified in Toxoplasma gondii (TgGCN5) contains a unique N-terminal “extension” that bears no similarity to known proteins and is devoid of known protein motifs. The hypothesis of this thesis is the N-terminal extension is critical to the function of TgGCN5. Three possible roles of the N-terminus were investigated: nuclear localization, protein-protein interactions, and substrate recognition. Subcellular localization was determined via immunocytochemistry using parasites expressing recombinant forms of TgGCN5 fused to a FLAG tag. Initial studies performed with parasites expressing full length FLAG-TgGCN5 were positive for nuclear localization. Without the N-terminal extension (FLAG-ΔNT-TgGCN5) the protein remains cytoplasmic. Additional studies mapped a six amino acid motif (RKRVKR) as the nuclear localization signal (NLS). When RKRVKR is fused to a cytoplasmic protein, it gains access to the nucleus. Furthermore, we have established the NLS interacts with Toxoplasma importin α, a protein involved in nuclear trafficking. Interaction with importin α provides evidence that the TgGCN5 N-terminal extension is involved in mediating protein-protein interactions. In order to identify additional interacting proteins, FLAG affinity purification was performed on parasites expressing full length FLAG-TgGCN5 and FLAG-ΔNT-TgGCN5. Upon comparing the results of the two purifications, proteins captured with only full length TgGCN5 may be interacting with the N-terminal extension. Full length TgGCN5 affinity purification indicates an interaction with histone proteins, two different homologues of Ada2 (adapter protein reported to interact with GCN5 homologues), and several heat shock proteins. With regard to substrate recognition, the N-terminal extension of TgGCN5 is dispensable for the acetylation of non-nucleosomal histones in vitro. However, the lysine acetylated by TgGCN5 is surprisingly unique. Other GCN5 homologues preferentially acetylate lysine 14 in histone H3, but TgGCN5 exclusively acetylates lysine 18 in histone H3 and has no activity on lysine 14. Taken together, these results argue that the N-terminal extension of TgGCN5 is critical for mediating protein-protein interactions, including those responsible for trafficking the HAT to the parasite nucleus but does not appear to be required for the acetylation of non-nucleosomal histones.