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GATE 2023 BT Q.21: solution and answer

GATE 2023 · BTQ.21 · 1 mark · MCQ

Intracellular proteins are targeted for proteolytic degradation in proteasomes upon conjugation with

  1. (A)

    ubiquitin

  2. (B)

    integrin

  3. (C)

    peptidase

  4. (D)

    calreticulin

See the question in the paper
GATE 2023 BT question 21Tap to open full size

Answer and solution

ANSWER

The correct answer is (A) ubiquitin.

Official answer: ASource: Official answer key, GATE 2023 BT (IIT Kanpur)

  1. Recall that intracellular protein turnover in eukaryotes proceeds mainly via the ubiquitin-proteasome system.

  2. Option (A) ubiquitin: a 76-residue protein covalently conjugated to lysine residues of a target protein via the E1-E2-E3 enzyme cascade; polyubiquitination is the signal recognized by the 26S proteasome for degradation. Correct.

  3. Option (B) integrin: a transmembrane cell-adhesion receptor linking the cytoskeleton to the extracellular matrix; not a degradation-targeting tag. Incorrect.

  4. Option (C) peptidase: an enzyme class that hydrolyzes peptide bonds; a protein is not "conjugated with" a peptidase as a targeting signal (proteasome subunits themselves act as peptidases, but that is not the tagging event). Incorrect.

  5. Option (D) calreticulin: an ER-lumen calcium-binding chaperone in the calnexin/calreticulin protein-folding quality-control cycle, not a proteasomal degradation tag. Incorrect.

  6. Therefore the molecule whose conjugation targets proteins for proteasomal degradation is ubiquitin, giving answer (A).

KEY IDEA

Covalent tagging of a target protein with (poly)ubiquitin chains is the signal recognized by the 26S proteasome for ATP-dependent proteolytic degradation.

WATCH OUT FOR

Confusing peptidase (an enzyme that carries out proteolysis, including proteasomal subunits) with the actual conjugated targeting tag; peptidase is never itself covalently attached to a substrate as a degradation signal.

From the GATE 2023 BT paper. Every question of this paper · Download the full paper · All 7 Cell signalling questions, with notes

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