Structure and function of the BAH-containing domain of Orc1p in epigenetic silencing

Abstract

contributed equally to this workThe N-terminal domain of the largest subunit of the Saccharomyces cerevisiae origin recognition complex (Orc1p) functions in transcriptional silencing and contains a bromo-adjacent homology (BAH) domain found in some chromatin-associated proteins including Sir3p. The 2.2 A Ê crystal structure of the N-terminal domain of Orc1p revealed a BAH core and a non-conserved helical sub-domain. Mutational analyses demonstrated that the helical sub-domain was necessary and suf®cient to bind Sir1p, and critical for targeting Sir1p primarily to the cis-acting E silencers at the HMR and HML silent chromatin domains. In the absence of the BAH domain,~14±20% of cells in a population were silenced at the HML locus. Moreover, the distributions of the Sir2p, Sir3p and Sir4p proteins, while normal, were at levels lower than found in wild-type cells. Thus, in the absence of the Orc1p BAH domain, HML resembled silencing of genes adjacent to telomeres. These data are consistent with the view that the Orc1p±Sir1p interaction at the E silencers ensures stable inheritance of pre-established Sir2p, Sir3p and Sir4p complexes at the silent mating type loci.

Meier-Gorlin syndrome mutations disrupt an Orc1 CDK inhibitory domain and cause centrosome reduplication

“…This is the same region of the BAH domain in the yeast Orc1-related Sir3 protein that binds to histone H4 present in a nucleosome ( Fig. 7B; Zhang et al 2002;Armache et al 2011). The histone H4 dimethylated peptide binds to an aromatic cage on one surface of the Orc1 BAH structure incorporating Tyr 63, Trp 87, Tyr 114, and Trp 119 (murine numbering) (red residues in Fig.…”

“…Of the mutations reported in human Orc1 in Meier-Gorlin syndrome, the three mutations (F89S, R105Q, and E127G) occurred within the CID that contains a bromo-associated homology (BAH) domain (Fig. 5A, top panel; Zhang et al 2002;Kuo et al 2012). Most Meier-Gorlin syndrome patients (seven out of 11 recently characterized) have an Orc1 with a mutation that changed an arginine at amino acid 105 to a glutamine (R105Q) (de Munnik et al 2012).…”

“…Modeling of the human Orc1 sequence based on the crystal structure of the Saccharomyces cerevisiae BAH domain of Orc1 (Zhang et al 2002) and the recent structure of the murine Orc1 BAH domain (Kuo et al 2012) suggests that the residues Arg 105 and Glu 127 are exposed on the surface of the human Orc1 CID structure, while Phe 89 is buried inside the structure (Fig. 7, below).…”

Structural basis of the Sir1–origin recognition complex interaction in transcriptional silencing

“…Analyses of HMR-E, a silencer that is necessary and sufficient to target silent chromatin to HMR, has established that the primary role for ORC in silencing is to recruit Sir1p to the silencer through direct protein-protein interactions between the Orc1p subunit of ORC and Sir1p (6)(7)(8)(9)(10)(11). ORC-bound Sir1p nucleates and͞or stabilizes a silent chromatin domain through direct protein-protein interactions with other Sir proteins, in particular Sir4p (7,11).…”

“…Molecular and genetic analyses have shown that the Sir1p͞ ORC interaction occurs through direct contacts between discrete domains within each protein. In ORC, the key interaction domain is the N-terminal bromo-adjacent homology (BAH) domain of Orc1p (10,13). BAH domains are conserved in Orc1 homologs across species (14) and are found in several other chromatin-associated proteins (15,16), suggesting that BAH domains play broad roles in regulating the structure and function of chromosomes.…”