Subtopic: Fundamentals of Downstream processing (chromatography, filtration)
Question Statement:
Which one of the following statements is true with regard to processing and presentation of
protein antigens?
Options:
(A) In the class II MHC pathway, protein antigens in the cytosol are processed by
proteasomes
(B) In the class I MHC pathway, extracellular protein antigens are endocytosed into vesicles
and processed
(C) In the class I MHC pathway, transporter associated antigen processing (TAP) protein is
required for translocating processed peptides generated in the cytosol
(D) Invariant chain in endoplasmic reticulum is involved in transport of peptides and loading
of class I MHC
Official Answer Key:
C
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2018 Q.43)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (2 Marks)
- **Topic**: Immunology (Antigen Processing & Presentation Pathways: MHC Class I vs. II)
**2. Detailed Step-by-Step Analysis**:
- In the **cytosolic (endogenous) pathway** for MHC Class I antigen presentation:
1. Endogenous or viral cytosolic proteins are tagged by ubiquitin and degraded into short peptide fragments (8–10 amino acids) by the **26S proteasome** (or immunoproteasome).
2. These cytosolic peptides must be transported across the endoplasmic reticulum (ER) membrane into the ER lumen.
3. This translocation is carried out by the ATP-dependent heterodimeric ABC transporter called **TAP (Transporter associated with Antigen Processing)**, composed of TAP-1 and TAP-2.
4. In the ER lumen, peptides are loaded onto newly assembled MHC Class I heavy chain-$eta_2$-microglobulin dimers via the peptide-loading complex (tapasin, calreticulin, ERp57).
- Therefore, **Option C** is biologically accurate and true.
- **Option-by-Option Evaluation**:
- **Option (A) [INCORRECT]**: In the MHC Class II pathway, *extracellular* protein antigens are endocytosed and degraded by *endosomal/lysosomal acid proteases* (cathepsins), not by proteasomes in the cytosol.
- **Option (B) [INCORRECT]**: Endocytosis of extracellular antigens is the hallmark of the MHC *Class II* pathway (except in cross-presentation).
- **Option (C) [CORRECT]**: In the MHC Class I pathway, TAP is strictly required for translocating cytosolic proteasomal peptides into the lumen of the ER.
- **Option (D) [INCORRECT]**: The invariant chain (Ii / CD74) is specifically involved in the assembly, ER export, and peptide-blocking of MHC *Class II* molecules, not MHC Class I.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **C**
- **Scoring Criteria**: +2.0 Marks (Negative marking: -0.67 marks).
Practice This in Real CBT Exam Interface
Test yourself with the official TCS-iON palette, virtual calculator, and immediate performance analytics.