Human NOC3 is essential for DNA replication licensing in human cells

From Flies to Humans: Conserved Roles of CEBPZ, NOC2L, and NOC3L in rRNA Processing and Tumorigenesis

NOC1, NOC2, and NOC3 are conserved nucleolar proteins essential for regulating ribosomal RNA (rRNA) maturation, a process critical for cellular homeostasis. NOC1, in Drosophila and yeast, enhances nucleolar activity to sustain rRNA processing, whereas its depletion leads to impaired polysome formation, reduced protein synthesis, and apoptosis. These genes have vertebrate homologs called CEBPZ, NOC2L, and NOC3l. In this study, we demonstrated that the RNA-regulatory functions of CEBPZ are conserved in vertebrates, and we showed that CEBPZ leads to the accumulation of 45S-pre rRNA, with consequent reduction in protein synthesis. Gene Ontology and bioinformatic analyses of CEBPZ, NOC2L, and NOC3L in tumors highlight a significant correlation between their reduction and the processes that regulate rRNA and 60S ribosomal maturation. Comparative analysis of their expression in tumor databases revealed that CEBPZ, NOC2L, and NOC3L exhibit contrasting expression patterns across tumor types. This dual role suggests that their overexpression promotes tumor growth, whereas reduced expression may exert tumor-suppressive effects, uncovering unexpected regulatory functions exerted by these proteins in cancer.

An essential Noc3p dimerization cycle mediates ORC double-hexamer formation in replication licensing

Replication licensing, a prerequisite of DNA replication, helps to ensure once-per-cell-cycle genome duplication. Some DNA replication-initiation proteins are sequentially loaded onto replication origins to form pre-replicative complexes (pre-RCs). ORC and Noc3p bind replication origins throughout the cell cycle, providing a platform for pre-RC assembly. We previously reported that cell cycle-dependent ORC dimerization is essential for the chromatin loading of the symmetric MCM double-hexamers. Here, we used Saccharomyces cerevisiae separation-of-function NOC3 mutants to confirm the separable roles of Noc3p in DNA replication and ribosome biogenesis. We also show that an essential and cell cycle-dependent Noc3p dimerization cycle regulates the ORC dimerization cycle. Noc3p dimerizes at the M-to-G1 transition and de-dimerizes in S-phase. The Noc3p dimerization cycle coupled with the ORC dimerization cycle enables replication licensing, protects nascent sister replication origins after replication initiation, and prevents re-replication. This study has revealed a new mechanism of replication licensing and elucidated the molecular mechanism of Noc3p as a mediator of ORC dimerization in pre-RC formation.

An Essential and Cell-Cycle-Dependent ORC Dimerization Cycle Regulates Eukaryotic Chromosomal DNA Replication

Eukaryotic DNA replication licensing is a prerequisite for, and plays a role in, regulating genome duplication that occurs exactly once per cell cycle. ORC (origin recognition complex) binds to and marks replication origins throughout the cell cycle and loads other replication-initiation proteins onto replication origins to form pre-replicative complexes (pre-RCs), completing replication licensing. However, how an asymmetric single-heterohexameric ORC structure loads the symmetric MCM (minichromosome maintenance) double hexamers is controversial, and importantly, it remains unknown when and how ORC proteins associate with the newly replicated origins to protect them from invasion by histones. Here, we report an essential and cell-cycle-dependent ORC "dimerization cycle" that plays three fundamental roles in the regulation of DNA replication: providing a symmetric platform to load the symmetric pre-RCs, marking and protecting the nascent sister replication origins for the next licensing, and playing a crucial role to prevent origin re-licensing within the same cell cycle.