Serine/threonine kinase which acts as an essential component of the MAP kinase signal transduction pathway. Plays an important role in the MAPK/ERK cascade. Depending on the cellular context, this cascade mediates diverse biological functions such as cell growth, adhesion, survival and differentiation through the regulation of transcription, translation, cytoskeletal rearrangements. The the MAPK/ERK cascade plays also a role in initiation and regulation of meiosis, mitosis, and postmitotic functions in differentiated cells by phosphorylating a number of transcription factors. Many of the substrates are localized in the nucleus, and seem to participate in the regulation of transcription upon stimulation. However, other substrates are found in the cytosol as well as in other cellular organelles, and those are responsible for processes such as translation, mitosis and apoptosis. Moreover, the MAPK/ERK cascade is also involved in the regulation of the endosomal dynamics, including lysosome processing and endosome cycling through the perinuclear recycling compartment (PNRC)\; as well as in the fragmentation of the Golgi apparatus during mitosis. Phosphorylates microtubule-associated protein 2 (MAP2), myelin basic protein (MBP), and Elk-1. Activated by M phase promoting factor (MPF).