# Mechanisms of SAGA Regulation by Histone Crosstalk

> **NIH NIH F32** · JOHNS HOPKINS UNIVERSITY · 2024 · $76,756

## Abstract

Project Summary:
The post-translational modification (PTM) of core histones is integral to the regulation of transcription. The
SAGA (Spt-ADA-GCN5-acetyltransferase) complex, capable of H3 acetylation and H2B deubiquitination, is a
transcriptional co-activator involved in nearly all Pol II mediated transcription. This 1.8mDa complex,
composed of 19 subunits in yeast, contains a core complex and two enzymatic complexes: the histone
acetyltransferase (HAT) module and deubiquitinating (DUB) module. The heterotetrameric HAT module,
composed of the acetyltransferase GCN5, ada2, ada3, and sgf29, contains multiple reader domains that bind
chromatin to acetylate histone tails. Increasingly, the SAGA HAT module is being recognized as a therapeutic
target in a multitude of c-myc driven cancers. While recent cryo-EM structures of yeast and human SAGA have
been reported, neither the DUB nor HAT modules could be resolved, presumably due to their high mobility. In
addition, none of these studies explored the structural engagement of these modules to chromatin.
Interestingly, the distance between the DUB and HAT modules strongly suggests that these modules could act
in concert on neighboring nucleosomes, thereby acetylating one while deubiquitinating the other. Despite
this, the vast majority of research on SAGA has been done on peptides and, in a more limited way, on single
nucleosomes. In Aim 1 of my studies, I plan to study the acetylation activity of SAGA complex on a library of
mono- and di-nucleosomes with biologically relevant PTMs to probe how these modules act in concert to
exert their enzymatic activities. More broadly, this will inform our understanding of how SAGA utilizes its
reader, writer, and eraser functions to regulate transcription. Additionally, no structure exists to date of the
SAGA HAT module, limiting our mechanistic understanding of how SAGA acts to promote active transcription.
In Aim 2 of my studies, I plan to solve the cryo-EM structure of the SAGA HAT module bound to both
unmodified and H3K4me3 modified nucleosomes. From biochemical data and other chromatin modifying
enzymes, I hypothesize that the PTM H3K4me3, recognized by sgf29, limits the conformational landscape of
the HAT module on a nucleosome for the processive acetylation of H3 histone tails. Altogether, our studies
will provide insight into SAGA complex regulation in the context of histone crosstalk and establish a new
paradigm for studying chromatin modifying enzymes. Furthermore, our research will provide the field with a
mechanistic understanding of how the SAGA HAT promotes active transcription and, feasibly, will provide new
therapeutic targets in c-myc driven cancers.

## Key facts

- **NIH application ID:** 10998572
- **Project number:** 1F32GM156005-01
- **Recipient organization:** JOHNS HOPKINS UNIVERSITY
- **Principal Investigator:** James Kemper Fields
- **Activity code:** F32 (R01, R21, SBIR, etc.)
- **Funding institute:** NIH
- **Fiscal year:** 2024
- **Award amount:** $76,756
- **Award type:** 1
- **Project period:** 2024-08-01 → 2026-07-31

## Primary source

NIH RePORTER: https://reporter.nih.gov/project-details/10998572

## Citation

> US National Institutes of Health, RePORTER application 10998572, Mechanisms of SAGA Regulation by Histone Crosstalk (1F32GM156005-01). Retrieved via AI Analytics 2026-05-28 from https://api.ai-analytics.org/grant/nih/10998572. Licensed CC0.

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