Question

A dual affinity nitrate transporter (NPF 6.3) operates as both a low- and high-affinity transporter. Its ability to switch between two affinity modes depends upon its:

• A. Redox state
• B. Nitration state
• C. Phosphorylation state
• D. Cellular location and interaction with other proteins

Hint:

Remember that CIPK23 is the primary kinase that regulates both potassium (\(\text{AKT1}\)) and nitrate (\(\text{CHL1}\)) transporters in plants via phosphorylation.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Plants must adapt to fluctuating soil nitrate concentrations using transport systems that can alter their transport kinetics.

Step 2: Detailed Explanation:
The nitrate transporter NPF6.3 (historically known as CHL1) is a dual-affinity transporter in Arabidopsis.
This single protein acts as a low-affinity transporter under high soil nitrate conditions, and switches to a high-affinity transporter under low nitrate conditions.
This functional switch is regulated by its phosphorylation state.
When nitrate levels are low, the protein kinase CIPK23 phosphorylates NPF6.3 at the Threonine-101 (T101) residue. This phosphorylation converts the protein into a high-affinity transporter.
When soil nitrate levels are high, the protein remains unphosphorylated, operating as a low-affinity transporter.

Step 3: Final Answer:
The affinity switch of the NPF 6.3 transporter is controlled by its phosphorylation state.