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Evolutionarily conserved developmental pathways

Cell cycle 1: Replication licensing factor (RLF) and Origin replication complex (ORC)

What proteins are needed to trigger initiation of DNA replication, and how do they intersect the other proteins and processes involved? Two sets of factors are required. One factor, called the origin recognition complex (ORC), recognizes origins of replications, and signals to the DNA replication apparatus where to carry out its function. Another factor 'licenses' DNA replication, that is, it gives the go ahead for initiation of DNA replication. Elements of both factors are highly conserved from yeast to mammals. Only a few of the Drosophila and mammalian genes have been cloned, so list of homologous proteins are yet incomplete. In yeast there are six MCM proteins and multiple ORC proteins. Drosophila disc proliferation abnormal (an MCM4 homolog) and Minichromosome maintenance 2 contain phosphorylation sites for cdc2 protein kinase, suggesting that these constituents of RLF are activated by G1-S cyclases, another evolutionarily conserved component regulating the cell cycle.

The Cdc6/18 protein has been mainly characterised for its role in the initiation of DNA replication (see Bell, 2002a for a review describing Origin recognition complex and CDC6 function in initiation of DNA replication; see ORC1 information of the role of ORC in DNA replication in Drosophila). Several studies exist, however, which suggest that it may also have a role in controlling the G2/M transition. Studies on the Drosophila Cdc6 (DmCdc6) protein support this dual function for the protein. Its location is consistent with a cellular role post replication initiation since it remains nuclear throughout G1, S and G2 phases. In addition, the level of DmCdc6 protein was reduced to nondetectable levels in S2 cells using RNAi. This causes DNA fragmentation and cell cycle abnormalities which have some similarities with phenotypes previously observed in yeasts and are consistent with the cells entering mitosis with incompletely replicated DNA. Finally, the DmCdc6 protein was stably overexpressed to a high level in S2 cells. Despite a large excess of protein, the effects on the S2 cells were minimal. However, a slight stalling of the cells in the late S phase of the cell cycle was detected, further supporting the proposal that DmCdc6 has a role in controlling the transition from the S to M phases of the cycle (Crevel, 2005).

The Cdc6/cdc18 protein is thought to have two important roles during the progression of the cell cycle. One is in the initiation of DNA replication, and the other is in checkpoint processes controlling the passage of the cell through the later stages of the cell cycle. Of the two, its role in DNA replication has been more extensively studied. In this case Cdc6 is one of the earliest proteins required and is involved in the formation of the prereplicative complex (see A model for the state of pre-replication chromatin and cell cycle regulation in human cells from Fujita, 2006). Together with cdt1 (Drosophila homolog; Double parked), Cdc6 binds after the hetero-hexameric origin recognition complex (ORC; see Drosophila Orc2), but its binding precedes and is needed for the loading of the six-membered minichromosome maintenance protein complex. In the absence of Cdc6 no prereplicative complex can be formed. The prereplicative complex then provides the platform from which bulk DNA synthesis is launched. Although this sequence of events has been well studied, the exact biochemical activities catalysed by these proteins remain to be clarified, with the exception of the MCM complex which is thought to be the replicative helicase (Crevel, 2005 and references therein).

Drosophila                   Homologs in other species ----------                   ----------------------------Orc1                         Yeast:  Orc1                             Vertebrates: Orc1Orc2                         Yeast:  Orc2                             Vertebrates: Orc2Cdc6                         Yeast:  Cdc18/CDC6                             Vertebrates: Cdc6Double parked                Yeast:  cdt1                              Vertebrates: cdt1Disc proliferation abnormal  Yeast: MCM4                             Vertebrates: MCM4

date revised: 13 Oct 96

Developmental Pathways conserved in Evolution

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