Metazoan Orc6 Proteins Evolved Alternative Mechanisms for Association with the ORC Complex: Insights from Drosophila Modeling

In eukaryotes, DNA replication requires the origin recognition complex (ORC), a six-subunit assembly that promotes replisome formation on chromosomal origins. Orc6 is the smallest and least evolutionarily conserved among all ORC subunits. In Drosophila, Orc6 binds tightly with the core ORC(1-5) and is required for DNA binding and replication initiation, whereas in Xenopus and human systems Orc6 loosely associates with the rest of the complex resulting in some differences for replication-associated activities. Despite these variations, Orc6 remains essential for viability in all species. In current study we analyzed specific residues within the C-terminal 11 helix that is critical for stable association of Orc6 with the ORC complex in Drosophila. Human Orc6 lacks these residues, however it possesses a strong nuclear localization signal (NLS) that is absent in Drosophilidae. We propose that this NLS drives human protein to the nucleus and compensates for weaker Orc6-ORC(1-5) interactions by increasing the nuclear concentration of Orc6 and shifting the equilibrium toward formation of the fully assembled ORC complex at the DNA.