What action do protein kinases perform in cellular signaling?

Protein kinases play a crucial role in cellular signaling by adding phosphate groups to proteins, a process known as phosphorylation. This modification can alter the activity, localization, or interaction of the target proteins, thereby regulating various biological processes within the cell. Phosphorylation often acts as a molecular switch, turning on or off the activity of enzymes and receptors, and influencing signaling pathways that control cell growth, differentiation, and response to external stimuli.

In contrast, the other choices do not accurately describe the primary function of protein kinases. Deactivation of signaling pathways typically involves mechanisms such as phosphatases, which remove phosphate groups, rather than the action of kinases. The removal of damaged proteins is generally carried out by proteases or cellular degradation pathways, not protein kinases. Transporting proteins across membranes is accomplished by distinct classes of proteins, such as transporters or channels, rather than through the action of kinases. Thus, adding phosphate groups to proteins is the defining action of protein kinases in the context of cellular signaling.