Transmembrane receptor Phosphorylation

The other activity associated with transmembrane receptors is phospholipase C. Phosphatidyl inositol is a membrane phospholipid that after phosphorylation on the head group is found in the membrane as a phos-photidylinostitol bis phosphate. Phospholipase C cleaves this into a membrane associated diacylglycerol (the lipid part) and inositol trisphosphate (IP3, the soluble part). Both play a later role in elevating the level of the second messenger, Ca2+. 

Fig. 6.4. Formation and function of diacylglycerol and Ins(l,4,5)P3. Formation of diacylglycerol (DAG) and Ins(l,4,5)P3 is subject to regulation by two central signaling pathways, which start from transmembrane receptors with intrinsic or associated tyrosine kinase activity (see Chapters 8 11) or from G-protein-coupled receptors. DAG activates protein kinase C (PKC, see Chapter 7), which has a regulatory effect on ceU proliferation, via phosphorylation of substrate proteins. Ins(l,4,5)P3 binds to corresponding receptors (InsPs-R) and induces release of Ca from internal stores. The membrane association of DAG, PtdIns(3,4)P2 and PL-C is not shown here, for clarity.

In the Akt signaling pathway (review Downward, 1998), first an extracellular growth factor activates the corresponding transmembrane receptor (e.g., PGDF receptor, see 8.1). Consequently, tyrosine phosphorylation takes place on the cytoplasmic domain of the receptor. The tyrosine residues serve as docking sites for the SH2 domain of the p85 subimit of the PI3-kinase. The associated translocation of PI3-kinase is synonymous with its activation. The PtdIns(3,4,5)P3 formed binds to the PH domain of the signal protein next in sequence, the Akt kinase, which recruits the latter to the membrane. 

For example, the growth factor receptor pathway involves an extracellular signal interacting with a transmembrane receptor. This then activates a tyrosine kinase causing phosphorylation of the receptor and allowing interaction with specific cellular proteins. This then sends a signal, which causes the expression of a particular gene. Oncoproteins alter this process. 

Cytokines all function using a group of transmembrane receptors embedded in the plasma membranes of target cells. The receptors have no tyrosine kinase activity but associate with and activate kinases known as Janus kinases (JAKs). These kinases phosphory-late tyrosine side chains in their receptors, and the phosphorylated receptors activate transcription factors of the STAT (signal transducer-activators of transcription) group.186-195 The specificity of cytokine action results from a combination of receptor recognition and recognition of the various STAT molecules by different JAKs.111 Cytokines have a variety of structures. Many are helix bundles or have (3 sheet structures (Fig. 30-6). 

Sibley DR, et al. Regulation of transmembrane signaling by receptor phosphorylation. Cell 1987 48(6) 913-922. 

Transforming growth factor (3 (TGF(3) (that suppresses cell proliferation), the related develop-mentally important activins (involved in mesoderm induction) and bone morphogenetic proteins (involved in bone formation) act via PM-located transmembrane receptors that are Ser/Thr-specific PKs. Thus, TGF(3 binds to the extracellular domain of a specific TGF(3 receptor with the successive consequences of activation of the receptor Ser/Thr-specific PK activity, phosphorylation of a protein Mad to yield P-Mad and downstream consequences resulting in developmentally important specific gene expression. Thus, dorso-ventral differentiation in Xenopus embryos involves Mad-like proteins and a Mad-like gene is a tumour suppressor gene. 

Although lipid modifications are believed to be necessary for bringing soluble, cytoplasmic proteins to their membrane-bound partners, it is not obvious why transmembrane receptors with structural features favouring their anchorage in the lipid bUayer must be acylated (palmitoylated). Therefore, lipid modifications of proteins may have other roles than attachment to membranes. In some cases lipid modification may change the conformation of proteins, just like phosphorylation an example is the Raf kinase, where the membranous environment activates the kinase conformationally (see Chapter 4). 

There seemed to be some hope of identifying such a receptor from the inside out. TNF, by this time taken as animportant endpoint of LPS responses, was synthesized as a result of separate transcriptional and translational activation events. Enhanced TNF gene transcription in myeloid cells followed LPS activation as a result of translocation of NF-/< B to the nucleus (40). Translational activation depended upon de-repression of a UA-rich element in the 3-untranslated region of the TNF mRNA (41). Subsequently, this event required activation of p38 (43), a protein that was first identified because it became phosphorylated in endotoxin-activated macrophages (44). In addition, LPS activated the MAP kinase pathway (45) and PI3 kinase pathway (46-49). The added importance of a tyrosine kinase in LPS signaling was suggested by the fact that tyrosine kinase inhibitors could block signal transduction (50). All attempts to find the critical transmembrane receptor that initiated these events were unsuccessful. 

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