O

PI 3,4,5-trisphosphate

PI 3,4-bisphosphate

PI 3,4,5-trisphosphate

▲ FIGURE 14-26 Generation of phosphatidylinositol 3-phosphates. The enzyme phosphatidylinositol-3 kinase (PI-3 kinase) is recruited to the membrane by many activated receptor tyrosine kinases (RTKs) and cytokine receptors. The 3-phosphate added by this enzyme is a binding site for various signal-transduction proteins. [See L. Rameh and L. C. Cantley, 1999, J. Biol. Chem. 274:8347.]

cruiting the enzyme phosphatidylinositol-3 kinase to the membrane. PI-3 kinase was first identified as a kinase that copurifies with several viral oncoproteins such as the "middle T" protein encoded by polyoma virus. When inactive, dominant negative, versions of PI-3 kinase are expressed in virus-transformed cells, they inhibit the uncontrolled cell proliferation characteristic of virus-transformed cells. This finding suggested that the normal kinase is important in certain signaling pathways essential for cell proliferation or for the prevention of apoptosis. Subsequent work showed that PI-3 kinases participate in many signaling pathways related to cell growth and apoptosis. Of the nine PI-3 kinase homologs encoded by the human genome, the best characterized contains a p110 subunit with catalytic activity and a p85 subunit with an SH2 domain.

The SH2 domain in PI-3 kinase binds to phosphotyro-sine residues in the cytosolic domain of many activated RTKs and cytokine receptors. The recruitment of PI-3 kinase to the plasma membrane by activated receptors positions its catalytic domain near its phosphoinositide substrates on the cytosolic face of the plasma membrane, leading to formation of PI 3,4-bisphosphate or PI 3,4,5-trisphosphate (Figure 14-26). By acting as docking sites for various signal-transducing proteins, these membrane-bound PI 3-phosphates in turn transduce signals downstream in several important pathways.

A primary binding target of PI 3-phosphates is protein kinase B (PKB), a serine/threonine kinase. Besides its kinase domain, PKB contains a PH domain that tightly binds the 3-phosphate in both PI 3,4-bisphosphate and PI 3,4,5-trisphosphate. In unstimulated, resting cells, the level of both these compounds is low, and protein kinase B is present in the cytosol in an inactive form. Following hormone stimulation and the resulting rise in PI 3-phosphates, protein kinase B binds to them and is localized at the cell surface membrane.

Binding of protein kinase B to PI 3-phosphates not only recruits the enzyme to the plasma membrane but also releases inhibition of the catalytic site by the PH domain in the cytosol. Maximal activation of protein kinase B, however, depends on recruitment of another kinase, PDK1, to the plasma membrane via binding of its PH domain to PI 3-phosphates. Both membrane-associated protein kinase B and PDK1 can diffuse in the plane of the membrane, bringing them close enough so that PDK1 can phosphorylate protein kinase B (Figure 14-27). PDK1 phosphorylates one serine residue in the activation lip of protein kinase B, providing yet another example of kinase activation by phos-phorylation in this segment. Phosphorylation of a second serine, not in the lip segment, is necessary for maximal protein kinase B activity. Thus, as with Raf, an inhibitory domain and phosphorylation by other kinases regulate the activity of protein kinase B. Once fully activated, protein ki-nase B can dissociate from the plasma membrane and phos-phorylate its many target proteins.

Exterior

PI 3,4-bisphosphate Cytosol

PI 3,4-bisphosphate Cytosol

PH domain

PH domain

Inactive PKB

Activation lip

Partially active PKB

Partially active PKB

Activation lip

Fully active PKB

Your Heart and Nutrition

Your Heart and Nutrition

Prevention is better than a cure. Learn how to cherish your heart by taking the necessary means to keep it pumping healthily and steadily through your life.

Get My Free Ebook


Post a comment