O

G protein-coupled receptors

Cytokine receptors

Receptor tyrosine kinases

TGFß receptors

Hedgehog (Hh) receptors

Wnt receptors

Notch receptor

G protein-coupled receptors

Cytokine receptors

Receptor tyrosine kinases

Linked to a trimeric G Associated with Cytosolic domain protein that controls cytosolic JAK kinases with tyrosine kinase the activity of an activity effector protein (here adenylyl cyclase)

TGFß receptors

Cytosolic domain with serine/threonine kinase activity

Activate cytosolic or Activate cytosolic Activate cytosolic Activate Smad nuclear transcription factors via several pathways (here one involving protein kinase A)

STAT transcription factors by phosphorylation kinases (here MAP kinase) that translocate to the nucleus and activate nuclear transcription factors by phosphorylation

Hedgehog (Hh) receptors

Hh ligand tethered to membrane of signaling cell by cholesterol anchor transcription factors in the cytosol by phosphorylation

Wnt receptors

Palmitoylated Wnt ligand binds seven transmembrane protein receptor complex

Notch receptor

Ligand, Delta, is a transmembrane protein on signaling cell

Control processing Release an activated Cytosolic domain of

Notch released by proteolysis acts in association with nuclear transcription factors of transcription factor transcription factor by proteolysis; from a multiprotein

Hh binding causes complex in the release from cytosol cytosolic complex

▲ FIGURE 13-1 Overview of seven major classes of cell-surface receptors discussed in this book. In many signaling pathways, ligand binding to a receptor leads to activation of transcription factors in the cytosol, permitting them to translocate into the nucleus and stimulate (or occasionally repress) transcription of their target genes. Alternatively, receptor stimulation may lead to activation of cytosolic protein kinases that then translocate into the nucleus and regulate the activity of nuclear transcription factors. Some activated receptors, particularly certain G protein-coupled receptors, also can induce changes in the activity of preexisting proteins. [After A. H. Brivanlou and J. Darnell, 2002, Science 295:813.]

process, is termed signal transduction. As we will see, signal-transduction pathways may involve relatively few or many components.

We begin this chapter with two sections that describe general principles and techniques that are relevant to most signaling systems. In the remainder of the chapter, we concentrate on the huge class of cell-surface receptors that activate trimeric G proteins. Receptors of this type, commonly called G protein-coupled receptors (GPCRs), are found in all eukaryotic cells from yeast to man. The human genome, for instance, encodes several thousand G protein-coupled receptors. These include receptors in the visual, olfactory (smell), and gustatory (taste) systems, many neurotransmitter receptors, and most of the receptors for hormones that control carbohydrate, amino acid, and fat metabolism.

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