Which of the following posttranslational modifications is defined by the addition of phosphates to a protein?

• A. Phosphorylation
• B. Ubiquitination
• C. Signal Sequences
• D. Protein Cleavage

Answer: (A)

Verified step by step guidance

1. Understand the concept of posttranslational modifications: These are chemical changes to a protein after it has been synthesized in a cell, which can affect the protein's function, activity, location, or stability.
2. Identify the specific modification in question: The problem asks about the addition of phosphates to a protein.
3. Recall the definition of phosphorylation: Phosphorylation is the process of adding a phosphate group (PO4) to a protein or other organic molecule, which is a common way to regulate the activity of proteins.
4. Differentiate between the options: Ubiquitination involves adding ubiquitin molecules, signal sequences are short peptides that direct the transport of a protein, and protein cleavage involves cutting the protein into smaller segments.
5. Conclude that phosphorylation is the correct term for the addition of phosphate groups to a protein, as it directly matches the definition provided in the problem.