The ADA Complex Is a Distinct Histone Acetyltransferase Complex in Saccharomyces cerevisiae

Abstract

We have identified two Gcn5-dependent histone acetyltransferase (HAT) complexes from Saccharomyces cerevisiae, the 0.8-MDa ADA complex and the 1.8-MDa SAGA complex. The SAGA (Spt-Ada-Gcn5-acetyltransferase) complex contains several subunits which also function as part of other protein complexes, including a subset of TATA box binding protein-associated factors (TAFIIs) and Tra1. These observations raise the question of whether the 0.8-MDa ADA complex is a subcomplex of SAGA or whether it is a distinct HAT complex that also shares subunits with SAGA. To address this issue, we sought to determine if the ADA complex contained subunits that are not present in the SAGA complex. In this study, we report the purification of the ADA complex over 10 chromatographic steps. By a combination of mass spectrometry analysis and immunoblotting, we demonstrate that the adapter proteins Ada2, Ada3, and Gcn5 are indeed integral components of ADA. Furthermore, we identify the product of the S. cerevisiae gene YOR023C as a novel subunit of the ADA complex and name it Ahc1 for ADA HAT complex component 1. Biochemical functions of YOR023C have not been reported. However,AHC1 in high copy numbers suppresses the cold sensitivity caused by particular mutations in HTA1 (I. Pinto and F. Winston, personal communication), which encodes histone H2A (J. N. Hirschhorn et al., Mol. Cell. Biol. 15:1999–2009, 1995). Deletion of AHC1 disrupted the integrity of the ADA complex but did not affect SAGA or give rise to classic Ada⁻ phenotypes. These results indicate that Gcn5, Ada2, and Ada3 function as part of a unique HAT complex (ADA) and represent shared subunits between this complex and SAGA.

ACKNOWLEDGMENTS

We thank members of the Workman lab for many helpful discussions. A.E. also thanks David Steger, Sam John, and Patrick Grant for their continuous encouragement during this work. We also thank LeAnn Howe for her help during the initial cloning steps.

This work was supported by the National Center for Research Resources, National Institutes of Health, and by grants from the National Institute of General Medical Sciences awarded to J.L.W., National Institutes of Health grant 11823-02 and NSF Science and Technology Center grant BIR9214821AM awarded to J.R.Y., and National Institutes of Health and NSF grants to S.L.B. A.E. was a recipient of a postdoctoral fellowship from the Austrian Science Foundation (Fonds zur Förderung der wissenschaftlichen Forschung, J1571-GEN) and was a postdoctoral associate of the Howard Hughes Medical Institute. D.E.S. was supported by an NIH postdoctoral fellowship. J.L.W. is an associate investigator of the Howard Hughes Medical Institute.