What is the role of Ras in tyrosine kinase signaling?

• A. Ras links the receptor protein to the MAP kinase pathway.
• B. Ras directly activates transcription factors.
• C. Ras binds GTP to inhibit the MAPK cascade.
• D. Ras is a receptor tyrosine kinase.

Answer: (A)

Small GTPases act as molecular switches that connect receptor activation to the MAP kinase signaling cascade. When a growth factor binds a receptor tyrosine kinase, the receptor becomes activated and recruits adaptor proteins like Grb2 and the guanine nucleotide exchange factor SOS. SOS promotes the exchange of GDP for GTP on Ras, turning it into the active form. Active Ras-GTP then engages and activates Raf, the first kinase in the MAPK cascade (Raf → MEK → ERK). This phosphorylation cascade leads to ERK entering the nucleus and regulating transcription factors that alter gene expression. Ras, therefore, links the receptor to the MAPK pathway; it does not directly activate transcription factors, and it is not a receptor tyrosine kinase nor an inhibitor of the pathway.