Certain regulatory molecules bind to membrane receptors that have an intracellular domain that is a tyrosine kinase. These receptors are comprised of two subunits that unite to form a dimer. Each subunit then phosphorylates the other. Regulatory molecules of this type tend to be growth factors linked to growth and development, rather than to responses hormones and neurotransmitters.
Once the receptor dimer is phosphorylated, an adaptor
protein binds and recruits a guanine nucleotide exchange protein to
the membrane. This protein in turn causes GTP to replace GDP on a monomeric
GTP binding protein termed Ras.
The Ras protein with GTP bound can now activate a protein kinase cascade, which ultimately leads to the phosphorylation of transcription factors in the nucleus, which in turn alter gene expression.
View this ANIMATION to see the sequence of events during the activation of a Ras protein.
See the lecture handout for connections between this system and oncogenes.
(OPTIONAL) If you like, click here to compare the conformations of the Ras protein with GDP and GTP bound. (You will need the free chime plug-in, which is available via the home page.)
