Q.2 Ras protein is a
(A) trimeric GTPase involved in relaying signal
from cell surface to nucleus.
(B) monomeric GTPase involved in relaying
signal from cell surface to nucleus.
(C) trimeric GTPase involved in regulation of
cytoskeleton.
(D) monomeric GTPase involved in regulation
of cytoskeleton.

Ras protein is a monomeric GTPase that relays signals from the cell surface to the nucleus. The correct answer is option (B).

Option Analysis

Ras functions as a small, single-subunit (monomeric) GTPase that cycles between GTP-bound (active) and GDP-bound (inactive) states to transmit signals from receptor tyrosine kinases at the cell surface through pathways like MAPK/ERK to the nucleus, regulating cell proliferation and differentiation.​

• (A) Trimeric GTPase involved in relaying signal from cell surface to nucleus: Incorrect, as Ras is monomeric, not trimeric (trimeric G proteins like Gαβγ handle some signaling but differ structurally and mechanistically from Ras).​

• (B) Monomeric GTPase involved in relaying signal from cell surface to nucleus: Correct, as Ras is a classic monomeric small GTPase activating downstream effectors like RAF for nuclear gene expression.​

• (C) Trimeric GTPase involved in regulation of cytoskeleton: Incorrect, as Ras primarily signals for growth/differentiation, not cytoskeleton (that’s Rho-family GTPases, which are also monomeric).​

• (D) Monomeric GTPase involved in regulation of cytoskeleton: Incorrect, though monomeric like Ras, its core role is signal transduction to the nucleus, not direct cytoskeletal control.​

Ras protein functions as a monomeric GTPase central to relaying signal from cell surface to nucleus, making it a key topic in molecular biology for competitive exams like IIT JAM. This Ras protein monomeric GTPase acts as a molecular switch in pathways controlling cell growth.

Ras Protein Structure and Activation

Ras protein, a small (~21 kDa) monomeric GTPase, binds GTP/GDP and localizes to the plasma membrane via farnesylation. Activation occurs when guanine nucleotide exchange factors (GEFs) like Sos promote GTP binding, enabling interaction with effectors like Raf kinase. GTPase-activating proteins (GAPs) then hydrolyze GTP to GDP, inactivating Ras for precise signaling control.​

Key Signaling Pathways

The Ras protein primarily activates the MAPK/ERK cascade: active Ras recruits Raf → MEK → ERK, culminating in nuclear transcription for proliferation and survival. It also engages PI3K/AKT for growth, but not cytoskeleton regulation (contrast with Rho GTPases). Mutations locking Ras in GTP-bound state (e.g., G12V) drive ~30% of cancers by constitutive nuclear signaling.​

Exam Relevance for IIT JAM

In questions distinguishing trimeric GTPase (e.g., G proteins) from monomeric GTPase like Ras, focus on Ras’s role in relaying signal from cell surface to nucleus versus cytoskeletal roles (Rho/Cdc42). Practice identifies (B) as correct by eliminating trimeric errors and cytoskeletal mismatches.​