ABSTRACT
Loading of the Mcm2-7 DNA replicative helicase onto origin-proximal DNA is a critical and tightly regulated event during the initiation of eukaryotic DNA replication. The resulting protein-DNA assembly is called the prereplicative complex (pre-RC), and its formation requires the origin recognition complex (ORC), Cdc6, Cdt1, and ATP. ATP hydrolysis by ORC is required for multiple rounds of Mcm2-7 loading. Here, we investigate the role of ATP hydrolysis by Cdc6 during pre-RC assembly. We find that Cdc6 is an ORC- and origin DNA-dependent ATPase that functions at a step preceding ATP hydrolysis by ORC. Inhibiting Cdc6 ATP hydrolysis stabilizes Cdt1 on origin DNA and prevents Mcm2-7 loading. In contrast, the initial association of Mcm2-7 with the other pre-RC components does not require ATP hydrolysis by Cdc6. Importantly, these coordinated yet distinct functions of ORC and Cdc6 ensure the correct temporal and spatial regulation of pre-RC formation.
Subject(s)
Adenosine Triphosphate/metabolism , Cell Cycle Proteins/metabolism , DNA Replication , DNA-Binding Proteins/metabolism , Fungal Proteins/metabolism , Nuclear Proteins/metabolism , Origin Recognition Complex/metabolism , Saccharomyces cerevisiae Proteins/metabolism , Adenosine Triphosphate/analogs & derivatives , Affinity Labels/metabolism , Cell Cycle/physiology , Cell Cycle Proteins/genetics , Chromosomal Proteins, Non-Histone , DNA-Binding Proteins/genetics , Fungal Proteins/genetics , Minichromosome Maintenance Complex Component 7 , Models, Biological , Nuclear Proteins/genetics , Saccharomyces cerevisiae/cytology , Saccharomyces cerevisiae/genetics , Saccharomyces cerevisiae/metabolism , Saccharomyces cerevisiae Proteins/geneticsABSTRACT
The origin recognition complex (ORC) is a six-subunit, ATP-regulated, DNA binding protein that is required for the formation of the prereplicative complex (pre-RC), an essential replication intermediate formed at each origin of DNA replication. In this study, we investigate the mechanism of ORC function during pre-RC formation and how ATP influences this event. We demonstrate that ATP hydrolysis by ORC requires the coordinate function of the Orc1 and Orc4 subunits. Mutations that eliminate ORC ATP hydrolysis do not support cell viability and show defects in pre-RC formation. Pre-RC formation involves reiterative loading of the putative replicative helicase, Mcm2-7, at the origin. Importantly, preventing ORC ATP hydrolysis inhibits this repeated Mcm2-7 loading. Our findings indicate that ORC is part of a helicase-loading molecular machine that repeatedly assembles Mcm2-7 complexes onto origin DNA and suggest that the assembly of multiple Mcm2-7 complexes plays a critical role in origin function.
Subject(s)
Adenosine Triphosphate/analogs & derivatives , Adenosine Triphosphate/metabolism , DNA Helicases/metabolism , DNA Replication/physiology , DNA-Binding Proteins/metabolism , Animals , DNA/metabolism , DNA-Binding Proteins/genetics , Humans , Mice , Mutation , Origin Recognition Complex , Saccharomyces cerevisiae/genetics , Saccharomyces cerevisiae/metabolism , Saccharomyces cerevisiae ProteinsABSTRACT
The Saccharomyces cerevisiae origin recognition complex (ORC) is composed of six subunits and is an essential component in the assembly of the replication apparatus. To probe the organization of this multiprotein complex by electron microscopy, each subunit was tagged on either its C or N terminus with biotin and assembled into a complex with the five other unmodified subunits. A nanoscale biopointer consisting of a short DNA duplex with streptavidin at one end was used to map the location of the N and C termini of each subunit. These observations were made using ORC free in solution and bound to the ARS1 origin of replication. This mapping confirms and extends previous studies mapping the sites of subunit interaction with origin DNA. In particular, we provide new information concerning the stoichiometry of the ORC-ARS1 complex and the changes in conformation that are associated with DNA binding by ORC. This versatile, new approach to mapping protein structure has potential for many applications.
