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Investigating the molecular architecture of yeast histone acetyltransferase complexes Setiaputra, Dheva


Post-translational modification of histones, such as the addition of acetyl groups, is a major regulatory mechanism for gene expression. Histone acetylation is catalyzed by highly conserved lysine acetyltransferase (KAT) enzymes that are often part of large, modular, and multifunctional complexes. Despite their fundamental importance, the reasons behind the tendency of these enzymes to form large complexes remain unclear. We investigated the organization of these complexes by elucidating the molecular architecture of three yeast KAT complexes: Spt-Ada-Gcn5 Acetyltransferase (SAGA), nucleosomal acetyltransferase of histone H4 (NuA4), and Elongator. The yeast SAGA complex is the largest KAT complex in yeast, and activates the expression of many stress response genes. Mutations of its human homologues have been implicated in spinocerebellar ataxia and oncogenesis. Using single particle electron microscopy and crosslinking coupled to mass spectrometry, we show that the catalytic module of SAGA resides within a highly flexible tail adjacent to numerous chromatin-binding subunits. We propose that the flexible SAGA tail is the nucleosome-interacting surface, and its plasticity serves to accommodate the various configurations of the chromatin substrate. NuA4 is another KAT complex whose catalytic subunit, Esa1, is the only essential KAT in yeast. NuA4 has highly conserved roles in the expression of housekeeping genes and the DNA damage repair pathway. Its subunits organize into modules that act independently of the complex. We show that these moonlighting modules form distinct globular structures that are peripherally associated with NuA4, which likely facilitates their dynamic nature. Similar to SAGA, NuA4 subunits that bind chromatin surrounds its catalytic subunit, possibly positioning its substrate nucleosome for efficient acetylation. Yeast Elongator, consisting of two copies each of six unique subunits, was initially characterized as a component of the elongating RNA polymerase II holoenzyme with histone acetyltransferase activity. However, further research has revealed a prominent role for the complex in modifying the wobble base pair of tRNAs. We generated the first three-dimensional reconstruction of Elongator and show that it organizes asymmetrically, with the two copies of its catalytic subunit residing in very different environments. Our structural investigations represent the first steps towards understanding the molecular mechanisms of these enigmatic complexes.

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