The reactions of cells to various internal and external stimuli are transmitted via cell signaling pathways. Transferring information from one location (exterior) to another (cytoplasm, nucleus) involves many components forming signal transduction cascades or pathways. Intracellular signal transduction is often accomplished through a series of reversible protein phosphorylation events. Protein kinases are enzymes that catalyze the transfer of the terminal phosphate group from ATP to a hydroxyl group of specific serine, threonine, or tyrosine residues on the downstream protein. Errors in these signaling pathways, caused by mutations that may result in constitutive expression, activation or inhibition of signaling proteins, are responsible for such diseases as diabetes, autoimmune disorders and cancer.
The cells ability to receive signals from its environment is essential for proper cell function.