Which subunit in MCM is not phosphorylated in MCM after initiation is triggered

In eukaryotic DNA replication, MCM (Mini-Chromosome Maintenance) proteins play a crucial role as helicases in unwinding the DNA double helix at replication origins. These proteins form a hexameric complex known as the MCM complex. During initiation of DNA replication, the MCM complex is recruited to replication origins and becomes phosphorylated as part of the activation process. However, after initiation is triggered, one subunit of the MCM complex, known as MCM2, is not phosphorylated.

MCM2 is one of the six subunits of the MCM complex, and it plays a central role in the function of the complex as a DNA helicase. During initiation of DNA replication, MCM2 along with other subunits of the MCM complex is phosphorylated by various protein kinases, including CDK (Cyclin-Dependent Kinase) and DDK (Dbf4-Dependent Kinase), which promotes the unwinding of the DNA double helix and the establishment of replication forks.

However, once initiation is triggered and DNA replication is underway, MCM2 remains unphosphorylated. This lack of phosphorylation of MCM2 after initiation ensures the stability and integrity of the replication fork and prevents premature termination of DNA replication. The unphosphorylated MCM2 subunit continues to function as part of the active helicase complex, facilitating the unwinding of the DNA template and the progression of the replication fork during DNA synthesis.