Signal transduction is the series of steps between the binding of a signal to a receptor and the ultimate cellular response. A protein kinase cascade amplifies a signal through a series of protein phosphorylation reactions. In many cases, a second messenger amplifies and distributes the downstream effects of the signal. The activity of transducer molecules is regulated.
learning outcomes
You should be able to:
Describe how a protein cascade amplifies a signal.
Explain how the same second-
Explain how signal transduction pathways are regulated in a cell.
How can an activated G protein such as ras amplify a response to ligand binding?
Amplification occurs because each activated ras molecule catalyzes the phosphorylation, and therefore activation, of many molecules of Raf, each of which in turn activates many molecules of MEK, etc. The general principle is one enzyme → many molecules of its substrate.
Cyclic AMP is a second messenger in many different responses. How can the same messenger act in different ways in different cells?
Different cells can have different target molecules to which cAMP binds, and these targets can have different activities and functions. Binding of cAMP changes the structure of its target (e.g., tertiary structure of a protein).
How are signal transduction cascades regulated?
Regulation of cascades occurs in three ways: by differing concentration of a second messenger (e.g., NO is a gas that breaks down readily), by differences in concentration of a single component of the pathway, and by enzymes that change target molecules (e.g., phosphatases that remove phosphate groups on proteins in a cascade).
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You have seen how the binding of a signal to its receptor initiates the response of a cell to the signal, and how signal transduction pathways amplify the signal and distribute its effects to numerous targets in the cell. In the next section we will look at the third step in the signal transduction process, the actual effects of the signal on cell function.