Question

Function of signal sequences in translation

• A. Protein modifications
• B. Protein targeting to other organelles for further protein synthesis
• C. Protein degradation
• D. Protein transport to target specific sites for function

Hint:

• Signal sequences are short amino acid stretches that act as "zip codes" or "address labels" for proteins.
• They direct newly synthesized proteins to their correct destinations (e.g., specific organelles, secretion pathway).
• This process is called protein targeting or protein sorting.

Answer 1

The Correct Option is D

A signal sequence (also known as a signal peptide, targeting sequence, or localization signal) is a short stretch of amino acids (typically 15-60 amino acids long) present at the N-terminus (or sometimes C-terminus or internally) of a newly synthesized polypeptide chain. The primary function of a signal sequence is to direct the protein to its correct location or destination within the cell or for secretion out of the cell. It acts as an "address label." Examples:

• Signal sequences can target proteins to the endoplasmic reticulum (for entry into the secretory pathway), mitochondria, chloroplasts, nucleus, peroxisomes, etc.
• Once the protein reaches its destination, the signal sequence is often cleaved off by a signal peptidase.

Let's analyze the options: (a) Protein modifications: While proteins undergo modifications, the signal sequence itself is for targeting, not the modification process itself (though it might be removed as a modification). (b) "Protein targeting to other organelles for further protein synthesis": Proteins are synthesized on ribosomes. Targeting to organelles is correct, but "for further protein synthesis" is not the general purpose; it's for the protein's function or further processing/folding in that organelle. (c) Protein degradation: Signal sequences are not primarily for degradation; specific signals (like ubiquitination tags) target proteins for degradation. (d) "Protein transport to target specific sites for function": This accurately describes the main role of signal sequences – directing proteins to their proper cellular or extracellular locations where they will perform their functions. Option (d) is the most comprehensive and correct statement about the function of signal sequences. \[ \boxed{\text{Protein transport to target specific sites for function}} \]