G
proteins – constitute a 3rd pathway
• Two classes,
heterotrimeric (containing α, β, and γ chains) and monomeric (also called small G-Proteins).
•Ras/RasV12 in cell growth; family members
•Bind GTP (active) and then cleave it to GDP (become
inactive) – they eventually
self-inactivate (mutation can make them oncogenic), are localized to the inner surface of the plasma membrane via
covalently attached lipids
•GEFs (guanine-nucleotide exchange factors) exchange GDP
for GTP, must be localized
to the inner surface of the membrane where the G-proteins reside
•Small G-proteins activate a cascade of protein kinases
known as the mitogen-activated
protein kinase pathway (MAP kinase pathway). This pathway is also widespread, and leads to transcription
of fos and jun which, in
turn, join together to form the AP-1 family of transcription factors.