<P> The first histone acetyltransferase to be described was GCN5, from the yeast species Saccharomyces cerevisiae. To date, the GCN5-related N-acetyltransferases (GNATs) comprise one of the largest enzyme superfamilies with over 10,000 identified members in sequenced genomes. This protein is known to acetylate specific lysine residues on the amino-terminal tails of nucleosomal histones, thereby loosening their contact with the tightly packed DNA and facilitating transcription. </p> <p> In this study, I determined that GCN5 is able to shuttle between the nucleus and the cytoplasm using fluorescence recovery after photobleaching (FRAP). Mutational studies revealed that its nuclear import is regulated by a classical bipartite nuclear localization signal (NLS) that is dependent on the transporters importin a and f3. In contrast, we found that GCN5 lacks a CRM1-dependent nuclear export signal (NES), as demonstrated by mutational and leptomycin B (LMB) studies; instead, IKB, a previouslydescribed transcription inhibitor with a CRMl-dependent NES, was found to modulate the export of GCN5 from the nucleus. This was initially discovered while performing the LMB assays, for which IKB served as a positive control, and was subsequently confirmed by mutational studies and protein complementation assays (PCAs). Furthermore, while the PCAs demonstrated a physical interaction between these two proteins in vivo, GST pull-down experiments were employed to confirm their interaction in vitro. </p> <p> Furthermore, this study also revealed that over-expression of GCN5-e YFP in NIH 3T3 cells causes -10% of the transfected cells to exhibit nuclear GCN5-eYFP-associated filaments; these structures were confirmed to be F -actin filaments comprised of f3-actin through co-localization studies with both TRITC-phalloidin and a mRFP-f-actin construct. GCN5's acetyltransferase activity was shown to be responsible for the formation ofthese filaments through mutation of its catalytic residue. Moreover, a protein complementation assay (PCA) demonstrated an in vivo interaction between GCN5 and f-actin, while FRAP analysis of a single filament showed that GCN5-e YFP molecules rapidly and randomly associate with these filaments along their entire length. Together these results suggest that GCN5's acetyltransferase activity is responsible for the structural maintenance of these filaments. Finally, GCN5-eYFP-associated filaments were found to be spatially separate from both lamin A (a nuclear envelope structural protein) and DNA; however, this does not exclude the possibility of an indirect interaction between these cellular constituents, as treatment of a live cell with Hoechst DNA stain, which disrupts the structure of DNA, was shown to disturb the structural integrity of these filaments. </p> / Thesis / Master of Science (MSc)

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