Summary

The identification of ras gene mutations in human cancers has focused a great deal of attention on the biological and biochemical function of the ras protein. A large number of experiments carried out both in tissue culture and in whole organisms has shown that ras is a key regulator of a signal transduction pathway linking plasma membrane receptors to cell growth and cell differentiation. Biochemically, ras is a GTP-binding protein and acts as a molecular switch; it is switched on by stimulated receptors that convert the protein from a resting GDP-bound state into an active GTP-bound state and once activated, it interacts with a (still unknown) downstream target to activate the MAP kinase cascade. Since MAP kinase can directly regulate a number of transcription factors, this now provides the basic outline of a ras-dependent signal transduction pathway that controls growth and differentiation.