Formation of noncanonical DNA structures mediated by human ORC4, a protein component of the origin recognition complex

Higher Order Oligomerization of the Licensing ORC4 Protein Is Required for Polar Body Extrusion in Murine Meiosis

We have previously shown that the DNA replication licensing factor ORC4 forms a cage around the chromosomes that are extruded in both polar bodies during murine oogenesis, but not around the chromosomes that are retained in the oocyte or around the sperm chromatin. We termed this structure the ORC4 cage. Here, we tested whether the formation of the ORC4 cage is necessary for polar body extrusion (PBE). We first experimentally forced oocytes to extrude sperm chromatin as a pseudo-polar body and found that under these conditions the sperm chromatin did become enclosed in an ORC4 cage. Next, we attempted to prevent the formation of the ORC4 cage by injecting peptides that contained sequences of different domains of the ORC4 protein into metaphase II (MII) oocytes just before the cage normally forms. Our rationale was that the ORC4 peptides would block protein-protein interactions required for cage formation. Two out of six tested peptides prevented the ORC4 cage formation and simultaneously inhibited PBE, resulting in the formation of two pronuclei (2 PN) that were retained in the oocyte. Together, these data demonstrate that ORC4 oligomerization is required to form the ORC4 cage and that it is required for PBE. J. Cell. Biochem. 118: 2941-2949, 2017. © 2017 Wiley Periodicals, Inc.

Human initiation protein Orc4 prefers triple stranded DNA

In higher eukaryotes mechanism of DNA replication origin recognition and binding by origin recognition complex (ORC) is still unknown. Origin transfer studies have shown that origin sites are genetically determined, containing functionally interchangeable modules. One of such modules from the human lamin B2 origin of replication has the ability to adopt unorthodox structure partly composed of intramolecular triplex. Sequences involved in triplex formation coincide with ORC binding sites both in vitro and in vivo. To explore potential significance of unorthodox DNA structures in origin recognition by ORC, we tested DNA binding properties of human ORC subunit 4 (HsOrc4) which has independent DNA binding activity in vitro and similar binding characteristics as ORC holocomplex. Our results demonstrated that DNA binding activity of HsOrc4 depends on length and structure of DNA with triplex being the protein's preferred binding target. Such feature could play part in origin selection through directing ORC to DNA sequence prone to adopt unorthodox structure.

The role of ATP in the function of human ORC4 protein

Human ORC4 protein, a subunit of the origin recognition complex, belongs to the AAA+ superfamily of ATPases. Proteins belonging to this family require ATP for their function and interactions with ATP lead to conformational changes in them or in their partners. Human ORC4 protein induces structural changes in DNA substrates, promoting renaturation and formation of non-canonical structures, as well as conversion of single-stranded into multi-stranded oligonucleotide structures. The aim of this study was to further investigate the role of ATP in the function of human ORC4 protein. For this purpose, a mutant in the conserved Walker B motif of ORC4, which is able to bind but not to hydrolyze ATP, was constructed and its activity in DNA restructuring reactions was investigated. The obtained results showed that ATP hydrolysis is not necessary for the function of human ORC4. It is proposed that ATP has a structural role as a cofactor in the function of human ORC4 as a DNA restructuring agent.