# Synthesis of a HDAC inhibitor–nanogold probe for cryo-EM visualization in class I HDAC co-repressor complexes

## TL;DR

Researchers created a new probe to visualize HDAC enzymes in co-repressor complexes using cryo-EM, but its positioning was limited in some views.

## Contribution

A novel HDAC inhibitor-nanogold probe was synthesized for cryo-EM imaging of class I HDAC co-repressor complexes.

## Key findings

- The nanogold probe retained HDAC inhibitory activity comparable to CI-994 in vitro.
- Cryo-EM 2D class averages showed the bi-lobed architecture of the CoREST complex.
- The probe localized partially to the CoREST complex but failed to identify the HDAC domain in side-view classes.

## Abstract

Class I histone deacetylases (HDACs 1–3) serve as catalytic subunits within seven multiprotein co-repressor complexes, each of which has distinct functions in the cell. We report the synthesis of a HDAC inhibitor–nanogold probe, derived from the class I HDAC inhibitor CI-994, for cryo-electron microscopy (cryo-EM) visualization of the HDAC catalytic domain within class I HDAC co-repressor complexes. The nanogold probe retained HDAC inhibitory activity comparable to CI-994 against the HDAC1-LSD1-CoREST complex in vitro. In cryo-EM studies, 2D class averages revealed the bi-lobed architecture of the CoREST complex and partial localization of the gold nanoparticle probe to the CoREST complex. However, the probe was not observed in classes showing the side-view of the CoREST complex, limiting unambiguous identification and positioning of the HDAC catalytic domain within the CoREST complex.