ORC proteins in the mammalian zygote

Abstract

The origin recognition complex (ORC) proteins, ORC1-6, are the first known proteins that bind DNA replication origins to mark the competency for the initiation of DNA synthesis. These proteins have complex mechanisms of assembly into the ORC complex and unexpected localizations in the mitotic chromosomes, cytoplasm, and nuclear structures. The mammalian zygote is a potentially important model that may contribute to our understanding of the mechanisms and features influencing origin establishment and in the identification of other functions of the ORC proteins. Together with expected localizations to the chromatin during G1, we found an unexpected distribution in the cytoplasm that appeared to accumulate ORC proteins suggesting potential roles for ORC subunits in mitosis and chromatin segregation. ORC1, 2, 3, and 5 all localize to the area between the separating maternal chromosomes shortly after fertilization. ORC4 forms a cage around the set of chromosomes that will be extruded during polar body formation before it binds to the chromatin shortly before zygotic DNA replication. These data suggest that the ORC proteins may also play roles in preparing the cell for DNA replication in addition to their direct role in establishing functional replication origins.

Keywords: DNA replication; Oocyte; Origin recognition complex; Sperm; Zygote.

Figure 1. Diagram of ORC Protein Localization during Oocyte Maturation, Fertilization and the First Zygotic Cell Cycle

All six of the ORC proteins have been localized during these stages of development. Dashed lines in B and C for ORCs 1, 3 & 5 represent suspected, but not proven localizations. See text for details.

Figure 2. Comparison of Mouse ORC4 and Drosophila ORC6

A septin-binding region was identified in DmORC6, and a similar region was found in MmORC4. MmORC4 also contained a much larger septin-like region.

Figure 3. A Model for the Role of MmORC4 in Polar Body Extrusion

Based on previous work on DmORC6 and our analysis of the MmORC4 sequence (Fig. 2) we suggest that MmORC4 interacts with the actin cap in the polar body through its septin-like domain.