Phosphorylation of Transcriptional activators

An example of how protein phosphorylation can influence the transcription process is the transition from the initiation to the elongation process for RNA polymerase II (see Section 1.4.3.4). 

Phosphorylation of transcriptional activators can influence the transcription activity according to the following mechanisms  

Proteins which act in the nucleus require specific sequences, known as nuclear localization sequences, to direct their transport from the cytoplasm to the nucleus. The nuclear localization sequences are generally found at the C-terminus of a protein and often comprise basic amino acids. Phosphorylation in sequences that are required for import into or export from the nucleus can decide whether the transcriptional activator is located predominantly in the cytoplasm or in the nucleus where it can exert its activating function. 

An example of this type of regulation is the SWI5 protein of yeast (Fig. 1.28). SWI5 is a transcriptional activator which up-regulates the expression of the HO endonuclease in yeast. SWI5 occurs in two different forms during the cell cycle  

The reason for the change in subcellular localization of SWI5 is phosphorylation in the region of the nuclear localization sequence. SWI5 possesses three sequences in the nuclear localization signal for phosphorylation at Ser and Thr residues. Cytoplasmi-cally localized SWI5 is phosphorylated at these positions, thus blocking transport into the nucleus. The protein thus remains in the cytosol. 

The phosphorylation of proteins on Ser, Thr or Tyr residues is a basic tool for the regulation of protein activity (see 7.1). Many eucaryotic transcriptional activators are isolated as phosphorylated proteins. The phosphorylation occurrs mainly on the Ser and Thr residues, but can also be observed on the Tyr residues. The extent of phosphorylation is regulated via specific protein kinases and protein phosphatases, each components of signal transduction pathways (see ch. 7). The phosphorylation of transcriptio- 

Phosphorylation in the nuclear localization sequence of transcriptional activators can decide whether transport into the nucleus, and subsequent transcription activation, occms or not. 

---

PRs also interact with other signaling pathways, which can, e.g., be regulated by phosphorylation. Independent of transcriptional activation of PR, progestins can activate cytoplasmic signaling molecules including SRC and downstream MAP kinase in mammalian cells via interaction by a specific polyproline motif in the N-terminal domain of PR. 

Direct Phosphorylation of Transcription Factors and Activation of STATs... 

Ali S, Metzger D, Bornert JM, Chambon P (1993) Modulation of transcriptional activation by ligand-dependent phosphorylation of the human oestrogen receptor A/B region. EMBOJ 12 1153... 

The effects of insulin on transcription are shown on the left of the illustration. Adaptor proteins Crb-2 and SOS ( son of sevenless ) bind to the phosphorylated IRS (insulin-receptor substrate) and activate the G protein Ras (named after its gene, the oncogene ras see p.398). Ras activates the protein kinase Raf (another oncogene product). Raf sets in motion a phosphorylation cascade that leads via the kinases MEK and ERK (also known as MARK, mitogen-activated protein kinase ) to the phosphorylation of transcription factors in the nucleus. 

The trans-activating domains of transcriptional activators are also common substrates for phosphorylation by protein kinases. The details by which the phosphorylation... 

Apart from direct activation via ligand binding, nuclear receptors are also subject to regulation by phosphorylation. Thus, transcriptional activity of PPARy can be regulated by growth factor stimulation via the mitogen-activated protein (MAP) kinase pathway (34). 

As indicated in Chapters 5, 7 and 8, transcription can be switched on by a variety of signalling pathways. Thus, cAMP-mediated pathways generate phosphorylated CREB proteins that activate expression of particular proteins by binding to promoters called cAMP response elements (GREs). Similarly, Ca2+-dependent PKC activation results in phosphorylation of transcription factors that bind to and activate tetradecanoylphorbol ester response element (TRE) promoters. In the immune response bacterial lipopolysaccharide (LPS) and particular cytokines can switch on signalling pathways resulting in activation of transcription factors such as NFkB (Chapter 7) with resultant expression of proteins such as cytokines,... 

CaMKIV is responsible for calcium-dependent gene transcription through the phosphorylation of transcription factors cAMP response element-binding protein (CREB), serum response factor (SRF), and myocyte enhancer factor-2 (MEF2). Oncoprotein 18 in the cytoplasm is also a substrate of CaMKIV. The activity of CaMKIV is activated by CaMKK and autophosphorylation. The elongation factor-2 is the only known substrate of CaMKIII (eEF-2K) at present. The phosphorylation of elongation factor-2 inhibits its functions. 

PKA stimulates the expression of specific genes by phosphorylating a transcriptional activator called the cAMP-... 

MAP kinase serine-threonine protein kinase an enzyme whose activity is stimulated by the action of mitogens and growth factors. Involved in phosphorylation of transcription factors and consequent stimulation of gene expression. 

Phosphorylation S10ph Transcriptional activation of early genes... 

The trans-activating domains of transcriptional activators are also common substrates for phosphorylation by protein kinases. The detailed mechanism by which the phosphorylation affects the interactions with the basal transcription apparatus is known in very few cases. The reason for this is the difficulty of identifying the specific interaction partner in the complex transcription apparatus. 

An important cytoplasmic substrate of ERK1/2 proteins is the 90 kDa ribosomal S6 kinase (RSK), also termed MAPK-activated protein kinase 1, MAPKAP-K1. This enzyme is activated by phosphorylation in the activation loop. A number of cellular functions of RSK have been proposed, including phosphorylation of transcription factors like CREB and NFkB as well as stimulation of protein biosynthesis by phosphorylation of the ribsomal protein S6. 

Majka, M., Ratajczak, J., et al. (2000). Binding of stromal derived factor-lalpha (SDF-lalpha) to CXCR4 chemokine receptor in normal human megakaryoblasts but not in platelets induces phosphorylation of mitogen-activated protein kinase p42/44 (MAPK), ELK-1 transcription factor and serine/threonine kinase AKT. Eur J Haematol 64(3) 164—72. 

---