46. With reference to the signaling pathway shown above, which one of the following options correctly identifies the intracellular components?
(1) a-Grb2, b-SOS, c=Ras-GTP, d=ERK
(2) a=GRB2, b-Ras-GDP. c=MEK, d=ERK
(3) a=SOS, b=Grb2, c=Ras-GTP, d=MAPK
(4) a=RTK, b=SOS, c=Ras-GDP, d=ERK

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Introduction

The MAP kinase (MAPK) signaling pathway is a pivotal cellular cascade regulating proliferation, differentiation, and survival. It is initiated by receptor activation and transmitted inside the cell through a series of specific intracellular proteins.

Key Intracellular Components

• Grb2: An adaptor protein that links activated receptor tyrosine kinases (RTKs) to the activation of Ras by recruiting SOS.

• SOS (Son of Sevenless): A guanine nucleotide exchange factor (GEF) that catalyzes the exchange of GDP to GTP on Ras, activating it.

• Ras-GTP: The active, GTP-bound form of Ras, a small G-protein that triggers the downstream kinase cascade.

• ERK (Extracellular signal-Regulated Kinase): A MAP kinase that, when phosphorylated and activated, enters the nucleus to regulate gene expression.

Correct Intracellular Protein Matching

Among the options listed, the correct sequence of intracellular components following receptor activation is:

1. a = Grb2

2. b = SOS

3. c = Ras-GTP

4. d = ERK

This sequence aligns with classical MAPK pathway signaling where Grb2 recruits SOS, SOS activates Ras, and Ras initiates a kinase cascade culminating in ERK activation.

Explanation of Other Options

• Option 2 mismatches early components (b = Ras-GDP is inactive), and excludes Grb2’s role.

• Option 3 reverses SOS and Grb2 positions.

• Option 4 identifies ‘a’ as RTK, which is a membrane receptor, not an intracellular adaptor.

Conclusion

Therefore, Option (1) “a=Grb2, b=SOS, c=Ras-GTP, d=ERK” correctly identifies the intracellular components of the MAP kinase signaling pathway.

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