Gene X always produces the same mRNA (one splice form) and the same protein in all cells in which it is expressed. The same amounts of gene X mRNA and protein are present in brain, kidney, and heart cells. However, the activity associated with the protein is present only in the heart. Which of the following are possible explanations? a. A splicing factor necessary for removing all of the primary transcript's introns is present only in the heart.
b. A splicing factor necessary for removing all of the primary transcript's introns is present only in the brain and kidney.
c. Protein X is phosphorylated only in the heart.
d. Protein X is phosphorylated only in the brain and kidney.

There are two explanations for this phenomenon: phosphorylation of the protein X in the heart activates its function (Option C) or phosphorylation of the protein X in the kidney and the brain deactivates its function (Option D). Therefore, both Option C and Option D are correct.

Phosphorylation is a posttranslational protein modification that is capable of activating or deactivating protein function. Phosphorylation alters the three-dimensional structure of a protein, thereby modifying its function (i.e., activate or deactivate the target protein). Phosphorylation occurs at specific amino acids (e.g., serine and threonine residues) of the target protein. Moreover, mRNA splicing (RNA processing) occurs before the protein state, it is a transcriptional process. This process consists in removing the non-coding regions or 'introns' from pre-mRNA and subsequent joining protein-coding regions or 'exons' to enable the translation of the mRNA into a protein. In consequence, in this case, it is possible that 1-phosphorylation of the protein X in the heart activates its function, or 2-phosphorylation of the protein X in the kidney and the brain deactivates its function. Therefore, both Option C and Option D are correct.

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