Ras protein is a
trimeric GTPase involved in relaying signal from cell surface to nucleus.
monomeric GTPase involved in relaying signal from cell surface to nucleus.
trimeric GTPase involved in regulation of cytoskeleton.
monomeric GTPase involved in regulation of cytoskeleton.
ANSWER
Ras protein is a monomeric GTPase that relays signal from the cell surface to the nucleus via the Raf-MEK-ERK (MAPK) cascade — option (B). (The official key also credited (D); see reasoning for why.)
Official answer: B or DSource: Official answer key, GATE 2020 BT (IIT Delhi)
Ras belongs to the small (monomeric, single-subunit) GTPase superfamily, distinct from the heterotrimeric G proteins that couple to seven-transmembrane GPCRs — so any option calling Ras 'trimeric' is wrong on that ground alone.
Ras is a molecular switch cycling between an inactive GDP-bound state and an active GTP-bound state. Growth-factor binding to a receptor tyrosine kinase (e.g. EGFR) causes GEFs (e.g. SOS) to load Ras with GTP.
Activated Ras-GTP binds Raf and triggers the Raf → MEK → ERK (MAPK) cascade, which translocates to the nucleus and alters gene transcription — this is Ras's textbook, hallmark function: relaying a signal from a cell-surface receptor to the nucleus.
Option A ('trimeric ... surface to nucleus'): the pathway description is right but 'trimeric' is factually wrong for Ras — eliminated.
Option B ('monomeric ... surface to nucleus'): matches both the correct oligomeric state and Ras's defining signalling role — this is the textbook-correct choice.
Option C ('trimeric ... cytoskeleton'): wrong on oligomeric state, and cytoskeletal regulation is the signature role of the Rho-family small GTPases (Rho, Rac, Cdc42), not of Ras itself — eliminated.
Option D ('monomeric ... cytoskeleton'): the oligomeric state is correct, but attributing cytoskeletal regulation as Ras's function (rather than surface-to-nucleus mitogenic signalling) does not match the standard textbook characterisation of Ras specifically — this option is defensible only if 'Ras protein' is read loosely as the whole Ras superfamily (which does include cytoskeleton-regulating Rho/Rac/Cdc42 members), which is a looser reading than the question intends.
The official key lists B OR D as acceptable, most likely because of exactly this superfamily-vs-specific-protein ambiguity in the stem; but under the standard, specific definition of 'Ras protein' taught in cell/molecular biology courses, option B is the precise and best-justified answer.
KEY IDEA
Ras is a small, monomeric GTPase (not a heterotrimeric G protein) that acts as a GDP/GTP-regulated molecular switch downstream of receptor tyrosine kinases, transmitting mitogenic signals from the cell surface to the nucleus through the Raf-MEK-ERK cascade.
WATCH OUT FOR
Confusing Ras (a monomeric/small GTPase) with heterotrimeric G proteins that couple to GPCRs, or conflating Ras's signalling role with the cytoskeleton-regulating role that belongs specifically to the Rho-family small GTPases (Rho, Rac, Cdc42) within the broader Ras superfamily.
From the GATE 2020 BT paper. Every question of this paper · Download the full paper · All 7 Cell signalling questions, with notes
