Final answer:
The specificity of protein targeting by PKA is achieved through recognition of specific amino acid sequences on target proteins, subcellular localization, scaffold proteins, and regulatory subunits. These factors ensure that only precise proteins are phosphorylated, leading to specific cellular responses.
Step-by-step explanation:
When Protein Kinase A (PKA) is activated, it phosphorylates specific target proteins to alter cell activity. This specificity is achieved through several mechanisms. Protein kinases recognize specific sequence motifs on target proteins, often involving the amino acids serine or threonine for PKA. Additionally, substrate-specificity is modulated by the subcellular localization of the kinase and its substrates, as well as by the presence of specific scaffold proteins that bring kinases and their substrates into proximity. Furthermore, regulatory subunits or domains within the kinases can influence substrate selection, ensuring only certain proteins are phosphorylated under the right conditions.
In the case of protein phosphorylation, the addition of a phosphate group from ATP to certain amino acid residues - serine, threonine, or tyrosine - causes a change in the protein's shape, which can activate or deactivate the protein's function. This process can be influenced by inhibitors, such as IK-B, which when phosphorylated by another kinase such as Protein Kinase C (PKC), cannot bind to NF-kB and therefore allows NF-kB to initiate RNA transcription in the nucleus. This illustrates how phosphorylation can regulate the activity of proteins and their inhibitors within cellular pathways.