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The origin recognition complex: a biochemical and structural view.

ABSTRACT: The origin recognition complex (ORC) was first discovered in the baker's yeast in 1992. Identification of ORC opened up a path for subsequent molecular level investigations on how eukaryotic cells initiate and control genome duplication each cell cycle. Twenty years after the first biochemical isolation, ORC is now taking on a three-dimensional shape, although a very blurry shape at the moment, thanks to the recent electron microscopy and image reconstruction efforts. In this chapter, we outline the current biochemical knowledge about ORC from several eukaryotic systems, with emphasis on the most recent structural and biochemical studies. Despite many species-specific properties, an emerging consensus is that ORC is an ATP-dependent machine that recruits other key proteins to form pre-replicative complexes (pre-RCs) at many origins of DNA replication, enabling the subsequent initiation of DNA replication in S phase.

SUBMITTER: Li H

PROVIDER: S-EPMC3779782 | biostudies-literature | 2012

REPOSITORIES: biostudies-literature

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The origin recognition complex: a biochemical and structural view.

Li Huilin H Stillman Bruce B

Sub-cellular biochemistry 20120101

The origin recognition complex (ORC) was first discovered in the baker's yeast in 1992. Identification of ORC opened up a path for subsequent molecular level investigations on how eukaryotic cells initiate and control genome duplication each cell cycle. Twenty years after the first biochemical isolation, ORC is now taking on a three-dimensional shape, although a very blurry shape at the moment, thanks to the recent electron microscopy and image reconstruction efforts. In this chapter, we outline ...[more]

PMID: 22918579

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Project description:The evolutionarily conserved Origin Recognition Complex (ORC), plays a key role in origin selection in eukaryotes. However, ORC is strikingly divergent in its DNA binding specificity, ranging from base-specific interactions in Saccharomyces cerevisiae to base-agnostic interactions in humans. The mechanisms underlying this distinct selectivity is unknown. Atomic model of the yeast ORC showed that base-specific interaction with the invariant thymines of the ARS consensus sequence (ACS) is encoded by a 19-amino acid insertion helix (IH) embedded in the winged helix domain (WHD) of Orc4. This IH is absent in the Orc4 of metazoans including humans, suggesting that removal of the IH might give the yeast ORC “human-like” properties. Indeed, yeast strain engineered with IH deficient Orc4 has completely altered ORC-binding sites enriched in poly-dT tracts located in larger nucleosome-depleted and intergenic open chromatin. In vivo and in vitro assays show that the mutant ORC loads MCM efficiently, in spite of its altered specificity in favor of binding patterns more characteristic of those observed in humans/metazoans. This work provides insights for understanding how ORC evolves to adopt a life cycle that requires plasticity in origin selection during development.

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Structural mechanism for replication origin binding and remodeling by a metazoan origin recognition complex and its co-loader Cdc6.

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A structural role for ATP in the formation and stability of the human origin recognition complex.

Project description:The locally restricted recruitment of the multisubunit origin recognition complex (ORC) to eukaryotic chromosomes defines the position of origins of DNA replication. In budding yeast and metazoans the DNA binding activity of ORC is stimulated by ATP and requires an AAA+-type nucleotide binding domain in the largest subunit. Little else is known about the mechanisms behind the ATP requirement for ORC in its initiator function and, specifically, the relevance of nucleotide binding domains present on other subunits. Here we show that ATP is required for specific subunit interactions in the human ORC, with the Orc4 subunit playing a critical role in this dynamic process. ATP is essential for the maintenance of ORC integrity and facilitates complex formation. Thus, besides its previously identified role in DNA binding, ATP serves also as a structural cofactor for human ORC.

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Project description:The origin recognition complex (ORC) has an important function in determining the initiation sites of DNA replication. In higher eukaryotes, ORC lacks sequence-specific DNA binding, and the mechanisms of ORC recruitment and origin determination are poorly understood. ORC is recruited with high efficiency to the Epstein-Barr virus origin of plasmid replication (OriP) through a complex mechanism involving interactions with the virus-encoded EBNA1 protein. We present evidence that ORC recruitment to OriP and DNA replication function depends on RGG-like motifs, referred to as LR1 and LR2, in the EBNA1 amino-terminal domain. Moreover, we show that LR1 and LR2 recruitment of ORC is RNA dependent. HMGA1a, which can functionally substitute for LR1 and LR2 domain, can also recruit ORC in an RNA-dependent manner. EBNA1 and HMGA1a RGG motifs bound to structured G-rich RNA, as did ORC1 peptides, which interact with EBNA1. RNase A treatment of cellular chromatin released a fraction of the total ORC, suggesting that ORC association with chromatin, and possibly cellular origins, is stabilized by RNA. We propose that structural RNA molecules mediate ORC recruitment at some cellular and viral origins, similar to OriP.

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