MAP kinase (also called extracellular-signal-regulated kinase) is a protein kinase that performs a crucial step in relaying signals from the plasma membrane to the nucleus.  An unusual feature of the MAP-kinases is that their full activation requires phosphorylation of both a threonine and a tyrosine, which are separated in the protein by a single amino acid.  The protein kinase that catalyzes both of these phosphorylations is called MAP-kinase-kinase.  The requirement for both tyrosine and threonine phosphorylation ensures that MAP-kinases are kept inactive unless specifiacally activated by MAP-kinase-kinase, whose only known substrates are MAP-kinases (Alberts et al., 1994).

Once activated, a MAP-kinase relays signals downstream by phosphorylating various proteins in the cell, including other protein kinases and gene regulatory proteins.  When activated, MAP- kinases migrate from the cytosol into the nucleus and phosphorylate Elk-1, thereby activating it to turn on the transcription of the fos gene (Alberts et al., 1994).

SOURCES
Alberts, B.; Bray, D.; Lewis, J.; Raff, M.; Roberts, K.; Watson, J.D. Molecular Biology of the Cell.  New York:
         Garland Publishing.  1994.
 

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