Characterization of the yeast trimeric-SAS acetyltransferase complex

Area of Science:

• Molecular Biology
• Epigenetics
• Yeast Genetics

Background:

• The yeast SAS2 gene encodes a MYST protein family histone acetyltransferase (HAT).
• SAS2 is the catalytic subunit of the SAS complex, involved in transcriptional silencing at yeast silent loci.
• While SAS complex's activity on free histones is known, its nucleosomal HAT activity was undocumented.

Purpose of the Study:

• To characterize the native yeast SAS complex's composition and enzymatic activity.
• To investigate the role of the acetyl-CoA binding motif in Sas2.
• To determine the specific histone targets and functional significance of SAS complex HAT activity.

Main Methods:

• Purification and molecular mass determination of the native yeast SAS complex.
• In vitro enzymatic assays using free histones and nucleosomes.
• Analysis of in vivo silencing function and histone acetylation patterns.
• Site-directed mutagenesis of the putative acetyl-CoA binding motif in Sas2.

Main Results:

• The native SAS complex is a trimeric complex of Sas2, Sas4, and Sas5 (approx. 125 kDa).
• SAS complex exhibits weak nucleosomal HAT activity, acetylating both free histones and nucleosomes.
• The acetyl-CoA binding motif in Sas2 is essential for both in vivo silencing and enzymatic activity.
• SAS complex specifically acetylates histone H4 lysine 16 (H4K16), unlike NuA4 which acetylates multiple lysines.

Conclusions:

• The SAS complex is a distinct MYST HAT with specific nucleosomal HAT activity.
• H4K16 acetylation by SAS complex is critical for its role in antagonizing the spread of silencing proteins (Sir proteins) at yeast silent loci.
• The findings elucidate the specific mechanism of SAS complex in regulating transcriptional silencing through targeted histone modification.