Abstract
MAP kinase is used to refer to at least two distinct proteins, known as the extracellular signal-regulated protein kinases ERK 1 and ERK2. The discussion of proteins that are regulated by ERKs points to the pleiotropic nature of signaling pathways regulated by this family of protein kinases. Relatively, few data are yet in hand to prove that any of these proteins are substrates for these enzymes in vivo. The impact of these enzymes on cell function can be deduced from the recent experiments using mutated enzymes. The ERKl phosphorylation site and lysine mutants have proved useful dominant inhibitors. In Jurkat cells, the mutants inhibit induction of the cytokine IL-2. It was found that ERKl and ERK2 mutants block the ability of ras, serum, and phorbol ester to induce transcription from a TPA response element. ERK2 mutants prevent proliferation caused by activated Raf EGF or small tau antigen. The ubiquitous MAP kinases are activated by a remarkable variety of hormones in differentiated cells and growth factors in dividing cells. Their activation has been linked to the transition from G0 to G1 in the cell cycle and to the induction of differentiated phenotypes. These enzymes are essential components of a universal protein kinase cascade implicated in the control of many cellular processes.
