Upstream binding factor phosphorylation

The interaction of NF-kB with IkB provides a wealth of examples of several different kinds of order-disorder processes. This work was started in our lab as a collaboration with Dr. E.A. Komives at the University of California, San Diego. Nuclear factor-kappaB (NF-kB) is a dimeric transcription factor widely employed for the transcription of stress-response genes, as it binds to kB upstream enhancer DNA sequences, where it recruits the transcriptional activator CBP. In an unstressed cell, the majority of the NF-kB resides in the cytoplasm, in complex with the inhibitor of NF-kB (IkB). Response to stress involves phosphorylation and ubiquitination of IkB and its subsequent degradation by the proteasome. The free NF-kB is transported to the nucleus, where it binds to the kB enhancer sequences and mediates the transcription of genes that include that of IkB, which acts subsequently to remove NF-kB from the DNA and return it to the cytoplasm as the NF-kB-IkB complex. 

A discrete region of 40 residues in the amino-terminal part of p38 MAPK is mainly responsible for screening of extracellular signals, transmitted by the upstream kinases, whereas the carboxy-terminal half of the AlAPK binds to downstream substrates, such as transcription factors. The amino-terminal recognition region of the MAPK contains an exposed a-helix in the proximity of the catalytic cleft, which is the phosphorylation site recognized by an upstream kinase. Phosphorylation opens the catalytic cleft and activates the kinase. This seems to be a common structural feature of kinases participating in sequential reactions in phosphorylation cascades. 

Figure 11,10, Very simple version of the RAS cell-signaJing pathway. Step 1. A hormone binds to its receptor in the plasma membrane. Step 2. Hormone binding provokes the RAS protein to bind to the receptor, and thus adhere to the inside of the plasma membrane. This event activates RAS. Step 3. The activation of RAS results in the phosphorylation of JUN, a transcription factor. The activated transcription factor OUN-phosphale) binds to special sequences of DMA upstream of certain genes, and activates the genes.

Phospholipases of type Cy are activated by receptor tyrosine kinases (see Chapter 8), and thus phospholipase Cy is involved in growth factor-controlled signal transduction pathways. The receptor tyrosine kinases (see Chapter 8) phosphorylate the enzyme at specific tyrosine residues and initiate activation of the enzyme. This activation mobilizes internal calcium stores and engages multiple protein kinase pathways. Characteristic for the structure of phospholipase Cy is the occurrence of SH2 and SH3 domains (see Chapter 8). These represent protein modules that serve to attach upstream and downstream partner proteins. The SH2 domains mediate binding to Tyr-phos-phates of the activated, autophosphorylated receptor tyrosine kinase. During this process, the phospholipase Cy enzymes are phosphorylated onTyr-residues and are thereby activated. 

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