MAP kinase kinase

MAP kinase kinase, also known as Mek, forms part of a phosphorylation cascade set off by by a signal molecule attaching to an enzyme-linked receptor e.g. receptor tyrosine kinase. To understand the significance of the MAP KK it is important to discuss it in the context of the whole signaling pathway. The signalling pathway is called a Ras-MAP-kinase signalling pathway. 

On binding of the signal molecule to the receptor, the receptor becomes activated by auto-phosphorylation as the receptor is phosphorylating an identical protein. Phosphate docking sites are then established on the receptor and these docking sites allow other signalling proteins to join. 

An adapter protein attached to the docking site phosphate of the receptor, and then a molecule called Ras GEF (Guanine Exchange Factor) docks onto the adapter protein. Ras is an example of a monomeric G-protein, it only has one unit. The GEF results in the activation of the Ras G-protein by the Ras releasing GDP and taking up a GTP. 

The signal is now being transduced and amplified and this is where MAP kinases' are involved. The activated Ras protein activates a MAP (Mitogen Acrivated Protein) kinase kinase kinase, also called Raf. This then activates a MAP kinase kinase then activates a map kinase also known as Erk, which activates various proteins such as transcription factors.