Question

Match List - I with List - II. Proteins and Function. 

Choose the correct answer from the options given below : 
 

• A. A-II, B-IV, C-I, D-III
• B. A-I, B-IV, C-II, D-III
• C. A-III, B-IV, C-I, D-II
• D. A-II, B-I, C-III, D-IV

Hint:

To differentiate late-stage regulators: CD59 works on the cell surface to block C9. S protein works in the fluid phase (blood serum) to block C5b67 membrane insertion.

Answer 1

The Correct Option is A

Step 1: Concept:
The question tests knowledge of the complement system, specifically the regulatory proteins that tightly control the complement cascade to prevent unnecessary damage to host cells.

Step 2: Step-by-step Explanation:

• A. CR1 (Complement Receptor 1 CD35): CR1 acts as an essential cofactor for Factor I-mediated cleavage of C3b and C4b. By doing so, it aggressively accelerates the decay of C3 convertases (C4b2a in classical, C3bBb in alternative pathways) and inherently blocks their formation.
Thus, A matches with II.

• B. C1 inhibitor (C1INH): This is a classic Serine protease inhibitor (serpin). It binds to and irreversibly inactivates the serine proteases C1r and C1s of the classical pathway (as well as MASPs in the lectin pathway), halting cascade initiation.
Thus, B matches with IV.

• C. CD59 (Protectin): CD59 is a membrane-bound regulatory protein on host cells. It acts late in the complement cascade by specifically binding to the C5b678 complex and sterically hindering the recruitment and polymerization of C9, thereby preventing the formation of the lethal Membrane Attack Complex (MAC) on host tissues.
Thus, C matches with I.

• D. S protein (Vitronectin): The S protein acts as a fluid-phase regulatory protein. It binds to the soluble C5b67 complex as it is forming in the serum, preventing this hydrophobic complex from randomly inserting into the lipid bilayers of nearby innocent bystander host cells.
Thus, D matches with III.

• Consolidating the matches yields A-II, B-IV, C-I, D-III.

Step 3: Final Answer:
The matching corresponds exactly to Option (A).