Question

Once mRNA encoding a particular protein reaches the cytoplasm, what are four mechanisms that can regulate the amount of the protein that is active in the cell?

Short answer

The four mechanisms include

• regulation of translation initiation;

• mRNA degradation;

• protein activation;

• protein degradation.

Regulation of translation and mRNA degradation occurs before the translation of mRNA into proteins, whereas protein activation and degradation occur after protein synthesis.

Step-by-step solution

Synthesis of protein

The genes that code for specific proteins are expressed in the nucleus of the cell. The information present on the gene is transcribed to form messenger ribonucleic acid (mRNA) through transcription in the nucleus of the cell.

The mRNA is then exported to the cytoplasm of the cell for translation into proteins. The proteins synthesized in the cell depend on four mechanisms that regulate the amount of active protein.

Regulation of translation initiation 

In some cases, the translation initiation of mRNAs is blocked by regulatory proteins. These proteins bind to the UTR (untranslated regions) of mRNA, thereby inhibiting the ribosome’s binding from initiating translation.

However, translation initiation depends on the activation and inactivation of protein factors in eukaryotic cells. This process is usually observed in mRNAs stored in the egg, whose translation begins after the activation of translation initiation factors.

mRNA degradation

The mRNA in bacteria cells degrades within a few minutes after they are synthesized. Thus, bacterial cell lacks the pattern of protein synthesis as present in a eukaryotic cell. However, eukaryotic mRNAs can survive for a much longer time, such as up to weeks.

The life span of an mRNA molecule depends on the nucleotide sequence in the UTR at the 3’ end of the mRNA molecule. Thus, mRNA degradation regulates the amount of protein present in the cell.

Protein activation

After proteins are synthesized through translation, they are processed to yield functional protein molecules, such as the conversion of pro-insulin into active insulin. Additionally, some proteins undergo chemical modifications to become functional.

Many other proteins, such as cell surface proteins, are sent to their target cell to become active. Thus, the activation of proteins also determines the amount of functional protein in the cell.

Protein degradation

The proteins are degraded regularly for the proper functioning of the cell. The protein degradation occurs when the cell attaches a small ubiquitin protein to the protein destined for degradation.

Proteosomes recognize the ubiquitin-tagged proteins, and then they are degraded. Thus, the number of active proteins in the cell is also regulated by the degradation of proteins.