Mutations on the surface of HDAC1 reveal molecular determinants of specific complex assembly and their requirement for gene regulation.

Histone deacetylase 1 and 2 (HDAC1/2) regulate histone acetylation as catalytic and structural components of six unique multiprotein complex families: SIN3, NuRD, CoREST, MIDAC, MIER, and RERE. Co-immunoprecipitation of HDAC1-Flag followed by mass spectrometry revealed that 92% of HDAC1 in mouse embryonic stem cells resides in three complexes, NuRD (49%), CoREST (28%), and SIN3 (15%). We compared the structures of MTA1:HDAC1 and MIDEAS:HDAC1 to identify critical binding residues on the surface of HDAC1. Surprisingly, a single mutation, Y48E, disrupts binding to all complexes except SIN3. Rescue experiments performed with HDAC1-Y48E in HDAC1/2 double-knockout cells showed that retention of SIN3 binding alone is sufficient for cell viability. Gene expression and histone acetylation patterns were perturbed in both Y48E and a second mutant cell line, HDAC1-E63R, indicating that cells require a full repertoire of the HDAC1/2 complexes to regulate their transcriptome appropriately. Comparative analysis of MTA1/HDAC1 and SIN3B/HDAC2 structures confirmed the differential modes of HDAC1 recruitment, with Y48 interacting with ELM2/SANT domain-containing proteins, but not SIN3. The E63R mutation shows markedly reduced binding to NuRD and MiDAC complexes but retains some CoREST binding. We provide novel molecular insights into the abundance, co-factors and assemblies of this crucial family of chromatin modifying machines.