The GATE Grind

GATE 2022 BT – Question 43

Genetics, Cellular and Molecular Biology · Cell Biology: Post-translational modifications, protein trafficking, cell death and autophagy, extracellular matrix · 2 marks · Multiple choice

The enzymes involved in ubiquitinylation of cell-cycle proteins are

  1. E1 and E2 only
  2. E1 and E3 only
  3. E1 and E4 only
  4. E1, E2 and E3

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Correct answer: (D) E1, E2 and E3

Explanation

Attaching ubiquitin to a protein, as is done to mark cell-cycle proteins (cyclins, securin) for degradation, takes three enzymatic steps:

1. **E1 (ubiquitin-activating enzyme):** activates ubiquitin using ATP.
2. **E2 (ubiquitin-conjugating enzyme):** receives the activated ubiquitin from E1.
3. **E3 (ubiquitin ligase):** recognises the specific substrate (in the cell cycle, for example the SCF complex and the APC/C) and transfers ubiquitin from E2 to it.

All three are needed, so the answer is **E1, E2 and E3** (D). (E4 enzymes only extend ubiquitin chains in special cases.)