Roles of Phosphorylation in DNA Replication

Phosphorylation of proteins by protein kinases and dephosphorylation by protein phosphatases represents one of the most common, versatile, and perhaps confusing regulatory mechanisms in eukaryotic cells. Many, perhaps most, proteins in the eukaryotic nucleus are phosphoproteins, among them the proteins involved in DNA replication and its control. The importance of protein phosphorylation as a regulatory mechanism lies in its ready response to intracellular or extracellular signaling, its reversibility, and its ability to act as a measuring device to translate gradual changes into a molecular switch thrown at a threshold level of phosphorylation of a key target protein. The net level of protein phosphorylation is determined by the balance between the activities of protein kinases and those of protein phosphatases (for review, see Cohen 1989; Hubbard and Cohen 1993). The activities of kinases and phosphatases themselves are regulated by their own phosphorylation state. To complicate matters further, the number of known protein kinases, phosphatases, and regulatory subunits is growing rapidly, and their specificity is not always predictable from the primary sequence of the substrate protein (see Moreno and Nurse 1990; Cegielska et al. 1994a). Moreover, the effect of phosphorylation on the activity of a given target protein usually depends on the exact sites that are modified and unmodified. Thus, to understand how protein phosphorylation regulates the activity of replication proteins, one must know which sites in a target protein are phosphorylated, how physiological signals affect phosphorylation at each site, and how this phosphorylation influences the protein’s activity. Knowledge of protein kinases or phosphatases that act at each phosphorylation site is also useful in elucidating the role of protein phosphorylation in regulating the target protein.