The origin recognition complex subunit, ORC3, is developmentally regulated and supports the expression of biochemical markers of neuronal maturation in cultured cerebellar granule cells

Spatial and temporal resolution of mORC4 fluorescent variants reveals structural requirements for achieving higher order self-association and pronuclei entry

The Origin Replication Complex (ORC), which is a multi-subunit protein complex composed of six proteins ORC1-6, is essential for initiating licensing at DNA replication origins. We have previously reported that ORC4 has an alternative function wherein it forms a cage surrounding the extruded chromatin in female meiosis and is required for polar body extrusion (PBE). As this is a highly unexpected finding for protein that normally binds DNA, we tested whether ORC4 can actually form larger, higher order structures, which would be necessary to form a cage-like structure. We generated two fluorescent constructs of mouse ORC4, mORC4-EGFP and mORC4-FlAsH, to examine its spatial dynamics during oocyte activation in live cells. We show that both constructs were primarily monomeric throughout the embryo but self-association into larger units was detected with both probes. However, mORC4-FlAsH clearly showed higher order self-association and unique spatial distribution while mORC4-EGFP failed to form large structures during Anaphase II. Interestingly, both variants were found in the pronuclei suggesting that its role in DNA licensing is still functional. Our results with both constructs support the prediction that ORC4 can form higher order structures in the cytoplasm, suggesting that it is possible to form a cage-like structure. The finding that FlAsH labeled ORC4 formed demonstrably larger higher order structures than ORC4-GFP suggests that ORC4 oligomerization is sensitive to the bulky addition of GFP at its carboxy terminus.

Nonreplicative functions of the origin recognition complex

Origin recognition complex (ORC), a heteromeric six-subunit complex, is the central component of the eukaryotic pre-replication complex. Recent data from yeast, frogs, flies and mammals present compelling evidence that ORC and its individual subunits have nonreplicative functions as well. The majority of these functions, such as heterochromatin formation, chromosome condensation, and segregation are dependent on ORC-DNA interactions. Furthermore, ORC is involved in the control of cell division via its participation in centrosome duplication and cytokinesis. Recent findings have also demonstrated a direct interaction between ORC and mRNPs and highlighted an essential role of ORC in mRNA nuclear export. Along with the growth of evolutionary complexity of organisms, ORC complex functions become more elaborate and new functions of the ORC sub-complexes and individual subunits have emerged.

Licensing of DNA replication, cancer, pluripotency and differentiation: an interlinked world?

Recent findings provide evidence for a functional interplay between DNA replication and the seemingly distinct areas of cancer, development and pluripotency. Protein complexes participating in DNA replication origin licensing are now known to have roles in development, while their deregulation can lead to cancer. Moreover, transcription factors implicated in the maintenance of or reversal to the pluripotent state have links to the pre-replicative machinery. Several studies have shown that overexpression of these factors is associated to cancer.

OsORC3 is required for lateral root development in rice

The origin recognition complex (ORC) is a pivotal element in DNA replication, heterochromatin assembly, checkpoint regulation and chromosome assembly. Although the functions of the ORC have been determined in yeast and model animals, they remain largely unknown in the plant kingdom. In this study, Oryza sativa Origin Recognition Complex subunit 3 (OsORC3) was cloned using map-based cloning procedures, and functionally characterized using a rice (Oryza sativa) orc3 mutant. The mutant showed a temperature-dependent defect in lateral root (LR) development. Map-based cloning showed that a G→A mutation in the 9th exon of OsORC3 was responsible for the mutant phenotype. OsORC3 was strongly expressed in regions of active cell proliferation, including the primary root tip, stem base, lateral root primordium, emerged lateral root primordium, lateral root tip, young shoot, anther and ovary. OsORC3 knockdown plants lacked lateral roots and had a dwarf phenotype. The root meristematic zone of ORC3 knockdown plants exhibited increased cell death and reduced vital activity compared to the wild-type. CYCB1;1::GUS activity and methylene blue staining showed that lateral root primordia initiated normally in the orc3 mutant, but stopped growing before formation of the stele and ground tissue. Our results indicate that OsORC3 plays a crucial role in the emergence of lateral root primordia.