Ras GTPases are molecular switches that transduce mitogenic signals by activating downstream effectors, particularly in the MAPK pathway, when GTP is bound. Previous results have led to the belief that Ras is only active when bound to the cytosolic face of the plasma membrane. Several isoforms of human Ras exist, including K-Ras, N-Ras, and H-Ras, but the biological need for this diversity has been unclear. “If all three lead to MAPK activation, why have so many different isoforms?” says Philips. It has been hypothesized that the isoforms are alternatively regulated based on their differential localization within specific microdomains of the plasma membrane. Now, there may be an additional explanation—subcellular localization to specific internal membranes.
Philips' group used fluorescent-tagged reporters to reveal the distribution of active Ras within cells. They showed that mitogens can activate H-Ras and N-Ras on the ER and Golgi as well as on the plasma membrane. K-Ras, however, was active only on the plasma membrane. Ras constructs targeted to different membrane compartments activate specific downstream signals based on their subcellular localization. Although Golgi-localized H-Ras strongly activates Erk and Akt, H-Ras on the ER more efficiently activates Jnk. These differences may allow therapeutic intervention targeted to specific isoforms, such as K-Ras, the most common isoform affected in lung and colon cancers.
Since vesicular transport is not necessary for mitogens to activate internal membrane-associated Ras, a signal must diffuse from the plasma membrane to the Golgi or ER. Philips hypothesizes that the signal may be a guanine nucleotide exchange factor that either moves from the plasma membrane or is activated at the ER or Golgi in response to a diffusible second messenger, such as Ca2+. ▪