Like other GTP-binding proteins, Ras functions as a molecular switch, cycling between two distinct conformational states—active when GTP is bound and inactive when GDP is bound (Movie 15.7). As discussed earlier for monomeric GTPases in general, two classes of signaling proteins regulate Ras activity by influencing its transition between active and inactive states (see Figure 15–8).
Ras guanine nucleotide exchange factors (Ras-GEFs) stimulate the dissociation of GDP and the subsequent uptake of GTP from the cytosol, thereby activating Ras. Ras GTPase-activating proteins (Ras-GAPs) increase the rate of hydrolysis of bound GTP by Ras, thereby inactivating Ras. Hyperactive mutant forms of Ras are number of activated resistant to GAP-mediated GTPase stimulation and are locked permanently in the
GTP-bound active state, which is why they promote the development of cancer.
But how do RTKs normally activate Ras? In principle, they could either activate a Ras-GEF or inhibit a Ras-GAP. Even though some GAPs bind directly (via their SH2 domains) to activated RTKs (see Figure 15–46A), it is the indirect coupling of the receptor to a Ras-GEF that drives Ras into its active state. The loss of function of a Ras-GEF has a similar effect to the loss of function of Ras itself. Activation of the other Ras superfamily proteins, including those of the Rho family, also occurs through the activation of GEFs. The particular GEF determines in which membrane the GTPase is activated and, by acting as a scaffold, it can also determine which downstream proteins the GTPase activates.
The GEF that mediates Ras activation by RTKs was discovered by genetic studies of eye development in Drosophila, where an RTK called Sevenless (Sev) is required for the formation of a photoreceptor cell called R7. Genetic screens for components of this signaling pathway led to the discovery of a Ras-GEF called Son-of-sevenless (Sos). Further genetic screens uncovered another protein, now called Grb2, which is an adaptor protein that links the Sev receptor to the Sos protein; the SH2 domain of the Grb2 adaptor binds to the activated receptor, while its two SH3 domains bind to Sos. Sos then promotes Ras activation. Biochemical and cell biological studies have shown that Grb2 and Sos also link activated RTKs to Ras in mammalian cells, revealing that this is a highly conserved mechanism in RTK signaling (Figure 15–47). Once activated, Ras activates various other signaling proteins to relay the signal downstream, as we discuss next.
Both the tyrosine phosphorylations and the activation of Ras triggered by activated RTKs are usually short-lived (Figure 15–48). Tyrosine-specific protein phosphatases quickly reverse the phosphorylations, and Ras-GAPs induce activated 0.1 0.05
Figure 15–47 How an RTK activates Ras. Grb2 recognizes a specific phosphorylated tyrosine on the activated receptor by means of an SH2 domain and recruits Sos by means of two SH3 domains. Sos stimulates the inactive Ras protein to replace its bound GDP by GTP, which activates Ras to relay the signal downstream.