Q.59 PIN proteins are plasma membrane-localized carrier proteins required for polar auxin transport in
plants. Four different carrier proteins are shown in the diagram below labeled P1-P4. Arrow indicates the
direction of auxin flow. Which among these is most likely to be a PIN protein?
(A) P 1
(B) P 2
(C) P 3
(D) P 4

Introduction

Polar auxin transport is a fundamental process in plant development, controlling cell elongation, tropic responses, embryogenesis, and organ formation. This directional movement of auxin is mediated by specialized plasma membrane carrier proteins, among which PIN proteins (PIN-FORMED proteins) play a central role.

In this GATE/CSIR-NET–type question, four membrane-localized carrier proteins (P1–P4) are shown in a diagram. Based on the direction of auxin flow, we are asked to identify which carrier is most likely a PIN protein.

Key Concept: What Defines a PIN Protein?

PIN proteins are:

• Auxin efflux carriers

• Asymmetrically (polarly) localized on the plasma membrane

• Responsible for directional (polar) auxin flow from one cell to another

• Typically localized on the basal or lateral side of plant cells, depending on tissue context

👉 Therefore, the carrier positioned on the membrane side from which auxin exits the cell in a defined direction is the most likely PIN protein.

Understanding the Diagram

From the diagram:

• Arrows clearly indicate the direction of auxin movement

• Auxin is moving from the upper cell toward the lower cell

• Directionality is maintained across cell boundaries

• The key PIN protein must be located at the efflux point where auxin exits one cell to enter the next

Correct Answer

✅ Correct Option: (C) P3

Explanation of All Options

Option (A) P1 ❌

• P1 is located on the upper membrane of the top cell

• Auxin movement at this site is not consistent with efflux toward the next cell

• Likely represents an influx carrier or non-polar transporter

• ❌ Not a PIN protein

Option (B) P2 ❌

• P2 appears on the lateral membrane

• The auxin arrow near P2 indicates movement opposite to polar auxin flow

• PIN proteins do not function as bidirectional or inward-facing transporters

• ❌ Incorrect

Option (C) P3 ✅ (Correct Answer)

• P3 is positioned at the basal side of the upper cell

• Auxin arrow clearly shows efflux from the cell through P3

• This location perfectly matches the known polar localization of PIN proteins

• Responsible for directional auxin export

• ✅ Most likely a PIN protein

Option (D) P4 ❌

• P4 is located at the lower region but does not correspond to the primary efflux direction

• Likely involved in secondary transport or intracellular cycling

• Does not define the main polarity of auxin transport

• ❌ Not the best choice

Final Answer Summary

Conclusion

PIN proteins are directionally localized auxin efflux carriers that establish and maintain polar auxin transport in plants. In the given diagram, P3 best fits the defining characteristics of a PIN protein due to its basal localization and alignment with auxin efflux direction.

📌 Correct Answer: P3