Licensing factor

Immediately after mitosis has finished the cell cycle starts again, entering G1 phase of the cycle. At this point protein synthesis of various products required for the rest of the cycle begins. Two of the proteins synthesised are called Cdc6 and Cdt1 and are only synthesised in G1 phase. These two together bind to the origin recognition complex (ORC), which is already bound at the origin and in fact never leaves these sites throughout the cycle. Now we have a so-called pre-replication complex, which then allows a heterohexameric protein complex of proteins MCM2 to 7 to bind. This entire hexamer acts as a helicase unwinding the double stranded DNA. At this point Cdc6 leaves the complex and is inactivated, by being degraded in yeast but by being exported from the nucleus in metazoans, triggered by CDK-dependent phosphorylation. The next steps included the loading of a variety of other proteins like MCM10, a CDK, DDK and Cdc45, the latter directly required for loading the DNA polymerase. During this period Cdt1 is released from the complex and the cell leaves G1 phase and enters S phase when replication starts.

From the above sequence we can see that Cdc6 and Cdt1 fulfill the role of licensing factors. They are only produced in G1 phase, in addition to which binding of all the proteins in this process excludes binding of additional copies. In this way their mode of action is limited to starting replication once, since once they have been ejected from the complex by other proteins, the cell enters S phase, during which they are not re-produced or re-activated. Thus they act as licensing factors, but only together. It has been suggested that the whole pre-replication complex be called the licensing factor since the whole is required for assembling additional proteins to initiate replication.

• Recent paper on licensing in human cells