Signaling pathway General structure

About 10-20% of all transmembrane proteins that are targeted to the ER and subsequently enter the secretory pathway are subject to post-translational modification with glycosylphosphatidyl-inositol (GPI). Proteins bearing the GPI anchor are involved in signal transduction, immune response, cancer cell invasion, and metastasis and the pathobiology of trypanosomal parasites. The structure of the GPI anchor has been analyzed for mammals, protozoa, and yeast. The general structure of the glycolipid structure is shown in Scheme 4. 

With its multiple second messengers and protein kinases, the phosphoinositide signaling pathway is much more complex than the cAMP pathway. For example, different cell types may contain one or more specialized calcium- and calmodulin-dependent kinases with limited substrate specificity (eg, myosin light chain kinase) in addition to a general calcium- and calmodulin-dependent kinase that can phosphorylate a wide variety of protein substrates. Furthermore, at least nine structurally distinct types of protein kinase C have been identified. 

Neurotrophin Receptors Neurotrophins bind to and activate a family of receptor tyrosine kinases called Trks (pronounced tracks ). (The general structure of receptor tyrosine kinases and the Intracellular signaling pathways they activate are covered In Chapter 14.) As shown In Figure 22-28, NGF binds to TrkA BDNF, to TrkB and NT-3, to TrkC. Binding of these factors to their receptors provides a survival signal for different classes of neurons. A second type of receptor called (NTR = neurotrophin receptor) also binds to neurotrophins, but with lower affinity. However, forms heteromulti-... 

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