Final answer:
The third step of the Signal Transduction Pathway involves a phosphorylation cascade where proteins are sequentially phosphorylated. This results in activation or inhibition of these proteins, often leading to the activation of a transcription factor like STAT protein, which drives the cellular response. Second messengers like cAMP also aid in signal propagation within the cell.
Step-by-step explanation:
The Third Step of Signal Transduction Pathway (STP)-
The third step of the Signal Transduction Pathway involves a series of enzyme phosphorylations commonly referred to as the phosphorylation cascade. Once the extracellular signal binds to a receptor and activates it, the receptor itself displays kinase activity, beginning the phosphorylation cascade.
Through this process, a series of proteins are phosphorylated, which can significantly alter their shape and function, subsequently activating or inactivating them. The last protein in this cascade typically acts as a transcription factor, such as the STAT protein, which enters the nucleus and mediates the transcription of specific genes, leading to a cellular response.
During this third step, second messengers play a crucial role. These small, non-protein molecules (like cAMP, Ca²⁺, DAG, and inositol) help propagate the signal inside the cell. An example of a G-Protein mediated signal transduction involves the conversion of ATP to cyclic AMP (cAMP) by the enzyme adenylate cyclase, which then activates Protein Kinase A (PKA) and consequentially leads to a cellular response.