ORC4 Plays Another Role Other Than Origin Licensing In The Mouse Zygote

Abstract

DNA replication origins in eukaryotes are strictly regulated so that no origin starts replication more than once in a single cell cycle. The origin recognition complex (ORC), consists of six subunits ORC1, ORC2, ORC3, ORC4, ORC5, and ORC6, plays an essential role in the initiation of DNA replication by binding to origin sequences. We found an unexpected role for ORC4 in the polar body formation while we were investigating the ORC proteins in the mouse zygotes. During oogenesis, the two meiotic divisions are asymmetric yielding two small polar bodies while the oocyte retains most of the cytoplasm. We found that ORC4 surrounds the chromosomes in both meiotic divisions; ORC4 surrounds the set of chromosomes that will eventually be discarded in the polar bodies, but not the chromosomes that segregate into the oocyte. ORC4 localizes around the future polar body chromosomes as sphere-like structure, which moves to cortex to form 1st and 2sd polar bodies at anaphase I and anaphase II, respectively. In the zygote at the G1 stage, ORC4 surrounds the nuclei of 1st and 2sd polar bodies, but is absent from the paternal or maternal pronuclei. The absence of ORC4 in the pronuclei was surprising because ORC4 is required for DNA replication in somatic cells. Moreover, we did find ORC2 in zygotic pronuclei, as expected. When the zygote entered mitosis, the ORC4 was absent from the chromosomes at metaphase, but appeared on both sets of separating chromosomes at telophase. At this point, the ORC4 in the polar body also migrated into the nuclei. We are currently conducting additional experiments to determine the functional roles of ORC4 in polar body formation. Our results suggest that ORC4 localization can be used to help identify the DNA strands that destined to be expelled in the polar body. They also suggest that ORC4 is modified during the first embryonic cell cycle from a protein that is primarily localized to cytoplasmic, perinuclear structure to one that binds to DNA.