Questions: Post-Translational Modifications

Question 1

A kinase phosphorylates a signaling protein, switching it to its active form. You then apply a potent phosphatase inhibitor to the cell. What is the most likely consequence for the signaling protein?

• A. The signaling protein becomes inactive more quickly because its substrate is depleted
• B. The signaling protein remains in its active phosphorylated state longer because the phosphatase cannot remove the phosphate group
• C. The kinase stops working because negative feedback prevents further phosphorylation
• D. Ubiquitination of the signaling protein is immediately triggered to compensate

Answer: (B)

Phosphorylation is reversible: kinases add phosphate groups; phosphatases remove them. Inhibiting the phosphatase removes the 'off switch,' locking the protein in its active state. This illustrates the kinase/phosphatase toggle that underlies signal transduction cascades — activity is determined not just by what modifications are added but by the balance between enzymes that add and remove them. Options 0 and 2 misunderstand the direction of the effect; option 3 introduces ubiquitination with no mechanistic basis.

Question 2

A cell needs to rapidly degrade a regulatory protein in response to a DNA damage signal. Which PTM most directly targets the protein for proteasomal destruction?

• A. N-linked glycosylation of asparagine residues
• B. Serine phosphorylation by a stress-activated kinase
• C. Lysine-48-linked polyubiquitin chain attachment
• D. Histone acetylation at lysine residues

Answer: (C)

K48-linked polyubiquitin chains (four or more ubiquitin molecules) are the canonical 26S proteasome-targeting signal. N-linked glycosylation occurs in the ER and affects folding and cell-surface recognition — not degradation. Serine phosphorylation can regulate activity and sometimes primes a protein for ubiquitination (a phosphodegron), but is not itself the degradation signal. Histone acetylation loosens chromatin — it is not a proteolytic signal at all. Understanding that ubiquitin chain linkage type determines function (K48 = degradation; K63 = signaling/repair) is essential.

Question 3

Once a protein's amino acid sequence is established at translation, its activity, localization, and stability are fully determined and cannot be altered by the cell.

• T. True
• F. False

Answer: False

This is the central misconception the topic addresses. PTMs profoundly and dynamically alter protein function beyond what the sequence alone encodes. The same protein can be active or inactive (phosphorylation), nuclear or cytoplasmic (phosphorylation, acetylation), stable or targeted for rapid degradation (ubiquitination), properly folded or misrouted (glycosylation) — all depending on its current modification state. PTMs are the cell's language for controlling when, where, and how proteins act, and that language is written after translation is complete.

Question 4

Ubiquitination typically marks a protein for destruction by the proteasome.

• T. True
• F. False

Answer: False

Ubiquitin is far more versatile than a simple death tag. While K48-linked polyubiquitin chains (four or more) target proteins to the 26S proteasome, other ubiquitin modifications serve entirely different functions: monoubiquitination regulates endocytosis and histone function; K63-linked chains coordinate DNA damage repair and NF-κB signaling without triggering degradation. The meaning of ubiquitin depends on whether it is mono or poly, and which lysine links the chain. This combinatorial versatility is part of why ubiquitination is one of the most widespread and functionally diverse PTMs in eukaryotic cells.

Question 5

Why do post-translational modifications expand the proteome's functional capacity far beyond what ~20,000 human genes alone could encode?

Model answer: A single gene encodes one amino acid sequence, but PTMs allow that protein to exist in many distinct functional states. For example, a protein with multiple phosphorylatable serines, acetylatable lysines, and ubiquitination sites can theoretically exist in hundreds of combinatorially distinct modification states, each with different activity, interaction partners, localization, or stability. Additionally, many PTMs are reversible and dynamically regulated in response to signals, so the same protein can switch between states in real time without new synthesis. This combinatorial, reversible covalent modification system creates functional diversity that the genome's 20,000 genes vastly underestimates.

The concept of the 'proteome being larger than the genome' is a direct consequence of PTMs. A landmark estimate suggested the human proteome contains over 1 million distinct protein species, orders of magnitude more than the ~20,000 protein-coding genes. PTMs, along with alternative splicing, are the primary mechanisms generating this diversity. Understanding PTMs is therefore central to understanding how a relatively small genome can encode the full complexity of cellular life.