# DBT JRF 2022 — Question 73

Exam: DBT JRF 2022 · Section: B · Topic: Cell Biology › Ligand-Gated Ion Channels & Signal Transduction · Difficulty: Medium

## Question

The ligand binding domain of a ligand gated channel (LGC) has a serine residue, phosphorylation of which is essential for ligand binding. By site directed mutagenesis it was replaced by aspartic acid. The likely consequences will be:

## Options

- **A.** LGC becomes non-functional
- **B.** LGC becomes constitutively active
- **C.** No change in function
- **D.** It will rapidly shift between active and inactive forms

## Answer

Option B

## Explanation

Constitutively active ligand-binding domain using Asp mutation: The question describes engineering a constitutively active ligand-gated channel (LGC) by replacing Serine with Aspartate in the ligand-binding domain. Rational: • Serine is a neutral amino acid with a hydroxyl group (−OH) that can be phosphorylated (adding a negative charge −PO₃²⁻). • Aspartate has a carboxylate group (−COO⁻) — carries a permanent negative charge, mimicking constitutive phosphorylation. • If ligand binding (or the active state) involves a charge interaction at this Ser, replacing Ser→Asp creates a permanent negative charge that mimics the ligand-bound (active) state. • The LGC is then constitutively 'open' or active without needing ligand. This phosphomimetic substitution strategy is widely used in cell signalling research to create constitutively active proteins. Answer: Asp mimics phosphoserine (same negative charge), creating a constitutively active LGC.

Source: https://mypyqbuddy.com/questions/the-ligand-binding-domain-of-a-ligand-gated-6a104cd5920734408bc0c758