- Upon prolonged illumination, activated rhodopsin does not activate transducing, hence the vision is impaired. This could be because of the following explanations:
A. Most of the activated rhodopsin gets phosphorylated and is unable to activate transducin.
B. Most of the activated rhodopsin gets dephosphorylated and is unable to activate transducin.
C. Arrestin further interacts with phosphorylated rhodopsin.
D. Arrestin further interacts with dephosphorylated rhodopsin.
Which of the above combinations is correct?
(1) A and C (2) B and D
(3) A and D (4) B and C
Phototransduction is a finely regulated process initiating vision by converting light into electrical signals through activated rhodopsin catalyzing transducin activation. However, under prolonged illumination, vision can be impaired due to molecular mechanisms that inactivate rhodopsin’s ability to activate transducin. This regulation is vital to avoid continuous hyperactivation and allow reset of the visual system.
Molecular Explanation of Impaired Transducin Activation in Prolonged Illumination
Four key concepts are involved in this phenomenon:
A. Phosphorylation of Activated Rhodopsin
After light activation, rhodopsin undergoes phosphorylation on multiple serine and threonine residues in its C-terminal region by rhodopsin kinase (GRK1). This modification reduces rhodopsin’s ability to activate transducin without disassembling phototransduction machinery.wikipedia+1
B. Arrestin Binding to Phosphorylated Rhodopsin
Following phosphorylation, arrestin preferentially binds to phosphorylated rhodopsin, preventing further interaction with transducin. This effectively quenches the photoactivation signal and contributes to the process of light adaptation.nature+1
C. Role of Dephosphorylation
Dephosphorylation is a late event during rhodopsin recovery when the pigment resets to its dark state, and it is not responsible for the failure to activate transducin during prolonged light. Thus, dephosphorylation as a cause is incorrect here.wikipedia
D. Summary of Interaction
-
Phosphorylated rhodopsin (not dephosphorylated) interacts with arrestin.
-
This interaction prevents further activation of transducin, halting the phototransduction cascade.
Evaluating the Statements
| Statement | Explanation | Correct/Incorrect |
|---|---|---|
| A | Most activated rhodopsin gets phosphorylated | Correct |
| B | Most activated rhodopsin gets dephosphorylated | Incorrect |
| C | Arrestin interacts with phosphorylated rhodopsin | Correct |
| D | Arrestin interacts with dephosphorylated rhodopsin | Incorrect |
Correct Combination
The correct combination based on these molecular events is:
A and C
Conclusion
During prolonged illumination, the inability of activated rhodopsin to activate transducin, leading to impaired vision, is chiefly due to phosphorylation of rhodopsin and subsequent arrestin binding to the phosphorylated form. Dephosphorylation states and their interactions with arrestin do not contribute to this process.
