Protein phosphorylation cascades — What features characterize this signal transduction mechanism?

Introduction:
Protein phosphorylation is the backbone of many signaling pathways. This question asks which statements correctly describe how phosphorylation based cascades are initiated and propagated at the cell surface and within the cytoplasm.

Given Data / Assumptions:

• Extracellular ligand binds a receptor at the plasma membrane.
• Receptor kinases or associated kinases transmit signals inward.
• Downstream substrates are activated by phosphorylation.

Concept / Approach:
Ligand binding to receptor tyrosine kinases (or receptors coupled to kinases) triggers autophosphorylation, creating docking sites for adaptors and enzymes. Kinase cascades (e.g., MAPK, Akt) amplify and diversify the signal. Phosphorylated proteins alter enzyme activity, localization, and gene expression to produce responses.

Step-by-Step Solution:

Verification / Alternative check:
Inhibition with kinase inhibitors (e.g., RTK inhibitors) blocks downstream phosphorylation and cellular responses, confirming the centrality of receptor kinases and phosphorylated substrates.

Why Other Options Are Wrong:

• Only nuclear receptors with no kinases: incorrect; nuclear receptors are ligand activated transcription factors and are not required for phosphorylation cascades at the membrane.

Common Pitfalls:
Assuming phosphorylation is limited to tyrosine residues. Many cascades also use serine/threonine kinases and phosphatases for dynamic control.

Final Answer:
All of the above.