Final answer:
The GTPase Sar1 is involved in COPII vesicle formation from the endoplasmic reticulum to the Golgi apparatus, while the GTPase ARF is associated with COPI vesicle formation functioning in retrograde transport within the Golgi. Inhibition of RAS G-protein GTPase activity leads to uncontrolled downstream signaling and is associated with cancer due to continuous cell proliferation.
Step-by-step explanation:
The GTPase Sar1 is involved in the process of vesicle formation from the endoplasmic reticulum (ER). Sar1 is critical for the initiation of COPII vesicle formation, which is responsible for transporting proteins from the ER to the Golgi apparatus.
The GTPase ARF (ADP-ribosylation factor) is involved in vesicle formation as well, but it participates in forming COPI vesicles which mediate retrograde transport from the Golgi back to the ER, and COPI vesicles also perform functions within the Golgi.
Regarding the RAS G-protein, when its GTPase activity is inhibited, as seen in certain cancers, the RAS protein remains in an active state bound to GTP. This failure to hydrolyze GTP into GDP leads to persistent downstream signaling, especially in the MAPK signaling pathway also known as the kinase cascade. The RAS protein normally activates RAF, which then phosphorylates MEK, leading to the phosphorylation of ERK. Once activated, ERK enters the nucleus and triggers cellular responses including gene expression that can promote cell proliferation. Thus, the inhibition of RAS GTPase activity is associated with uncontrolled cell proliferation and tumorigenesis.