Page 236 - Veterinary Immunology, 10th Edition

VetBooks.ir Signal Transduction

When a cytokine binds its receptor, the receptor transmits a signal
to the cell to modify its behavior. This conversion of an extracellular

signal into a series of intracellular events is called signal
transduction. The key components of signal transduction include
binding of an agonist such as a cytokine to its receptor, possibly
clustering of receptor chains, activation of kinases by the receptor,
secondary activation of second messengers, generation of new

transcription factors, and gene activation leading to altered protein
synthesis and cell behavior (see Fig. 8.4). Because cell signaling
must be fast and precise, this is best accomplished by enzyme

cascades. Since enzymes can produce or modify a large number of
molecules in a short time, a pathway that involves the use of several
enzymes in sequence can amplify responses very rapidly.

Protein Phosphorylation

Central to most cell signaling is the reversible modification of
proteins by addition of a phosphate group to selected amino acids.
Signal transduction systems use a high-energy phosphate-rich
compound such as adenosine triphosphate (ATP) to modify

proteins and hence send a signal to a cell. Cell growth, cell division,
and other critical processes are all regulated by protein
phosphorylation. The enzymes that do this, called protein kinases,
enzymatically phosphorylate the amino acids serine, threonine, or

tyrosine.

In some proteins, only one amino acid is phosphorylated; in
others, multiple amino acids are phosphorylated. Phosphorylated

and nonphosphorylated proteins have different functional
properties. For example, the phosphorylation of serine or threonine
activates some enzymes, whereas dephosphorylation has the

opposite effect. In most cells, about 90% of the phosphate groups

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