Translation phosphorylation

Figure 4.12 Amino acid sequence of bovine k-casein, showing the amino acid substitutions in genetic polymorphs A and B and the chymosin cleavage site, Sites of post-translational phosphorylation or glycosylation are italicized (from Swaisgood, 1992).

Bernard, H., Meisel, H., Creminon, C., and Wal, J.M. 2000. Post-translational phosphorylation affects the IgE binding capacity of caseins. FEBS Lett 467(2-3) 239-244. 

Fig. 5. Hypothesis for the role of phosphorylation in the stimulation of steroidogenesis by ACTH. ACTH activation of cyclic AMP-dependent protein kinase results in the co-translational phosphorylation of protein pb to protein ib, postulated to be the effector of the increased availability of cholesterol to mitochondrial cytochrome P-450sc c. pa and i., are hypothesized to be derived from pb and ib. From Ref. 21.

Narayanan, A., Jacobson, M.R Computational studies of protein regulation by post-translational phosphorylation. Curr. Opin. Struct. Biol. 2009,19,156-63. 

SR-proteins can be expressed as recombinant proteins in Escherichia coli, but they lack the post-translational phosphorylation of the serine residues and are poorly soluble in the absence of chaotropic reagents. It is possible to phosphorylate bacterially produced protein by preincubation in nuclear extract or, even more efficiently, by the addition of purified recombinant SR-protein specific kinases Clk/Sty8 or SRPK.9 Soluble and phosporylated SR-proteins can also be produced in insect cells using the baculovirus system10 although it is not resolved whether these proteins behave exactly as those produced in human cells. 

Both ChEs undergo several post-translational modifications, including glycosylation and glycosylphosphatidy-linositolation (GPI), phosphorylation and carbamylation. 

After their synthesis (translation), most proteins go through a maturation process, called post-translational modification that affects their activity. One common post-translational modification of proteins is phosphorylation. Two functional classes of enzymes mediate this reversible process protein kinases add phosphate groups to hydroxyl groups of serine, threonine and tyrosine in their substrate, while protein phosphatases remove phosphate groups. The phosphate-linking... 

S6K1 (also known as p70S6 kinase) is a serine/ threonine protein kinase which is involved in the regulation of translation by phosphorylating the 40S ribosomal protein S6. Insulin and several growth factors activate the kinase by phosphorylation in a PI 3-kinase dependent and rapamycin-sensitive manner. Phosphorylation of S6 protein leads to the translation of mRNA with a characteristic 5 polypyrimidine sequence motif. 

When considering the role of phosphorylation in the regulation of the HS response, it is indeed curious that oxidative stress and heat induce a protein tyrosine phosphatase at the transcriptional level (Keyse and Emslie, 1992). Whether this phosphatase has any role in the regulation of HSF phosphorylation is not known, but it does indicate that both transcriptional and translational regulation of signaling... 

The activity of 4E is regulated in a second way, and this also involves phosphorylation. A recently discovered set of proteins bind to and inactivate 4E. These proteins include 4E-BP1 (BPl, also known as PHAS-1) and the closely related proteins 4E-BP2 and 4E-BP3. BPl binds with high affinity to 4E. The [4E] [BP1] association prevents 4E from binding to 4G (to form 4F). Since this interaction is essential for the binding of 4F to the ribosomal 40S subunit and for correctly positioning this on the capped mRNA, BP-1 effectively inhibits translation initiation. 

Recent evidence indicates that the 5-HT transporter is subject to post-translational regulatory changes in much the same way as neurotransmitter receptors (Blakeley et al. 1998). Protein kinase A and protein kinase C (PKC), at least, are known to be involved in this process. Phosphorylation of the transporter by PKC reduces the Fmax for 5-HT uptake and leads to sequestration of the transporter into the cell, suggesting that this enzyme has a key role in its intracellular trafficking. Since this phosphorylation is reduced when substrates that are themselves transported across the membrane bind to the transporter (e.g. 5-HT and fi -amphetamine), it seems that the transport of 5-HT is itself linked with the phosphorylation process. Possibly, this process serves as a homeostatic mechanism which ensures that the supply of functional transporters matches the demand for transmitter uptake. By contrast, ligands that are not transported (e.g. cocaine and the selective serotonin reuptake inhibitors (SSRIs)) prevent the inhibition of phosphorylation by transported ligands. Thus, such inhibitors would reduce 5-HT uptake both by their direct inhibition of the transporter and by disinhibition of its phosphorylation (Ramamoorthy and Blakely 1999). 

Fig. 10. Mechanisms of steady-slqte kinetics of sugar phosphorylation catalyzed by E-IIs in a non-compartmentalized system. (A) The R. sphaeroides 11 model. The model is based on the kinetic data discussed in the text. Only one kinetic route leads to phosphorylation of fructose. (B) The E. coli ll " model. The model in Fig. 8 was translated into a kinetic scheme that would describe mannitol phosphorylation catalyzed by Il solubilized in detergent. Two kinetic routes lead to phosphorylation of mannitol. Mannitol can bind either to state EPcy, or EPpe,. E represents the complex of SF (soluble factor) and 11 and II in A and B, respectively. EP represents the phosphorylated states of the E-IIs. Subscripts cyt and per denote the orientation of the sugar binding site to the cytoplasm and periplasm, respectively. PEP, phosphoenolpyruvate.

Post-translational modification of proteins plays a critical role in cellular function. For, example protein phosphorylation events control the majority of the signal transduction pathways in eukaryotic cells. Therefore, an important goal of proteomics is the identification of post-translational modifications. Proteins can undergo a wide range of post-translational modifications such as phosphorylation, glycosylation, sulphonation, palmitoylation and ADP-ribosylation. These modifications can play an essential role in the function of the protein and mass spectrometry has been used to characterize such modifications. 

As described in more detail below, agonist binding will lead to signaling as well as phosphorylation of Ser and Thr residues, especially, but also, in selected cases, Tyr residues located in intracellular loop-3 and in the C-terminal extension. This post-translational modification alters the affinity of the receptor for various intracellular proteins, including arrestin, which sterically prevents further G-protein binding and functions as an adaptor protein. Also, interaction with other types of scaffolding proteins such as PSD-95-like proteins, is influenced by the phosphorylation state of the receptor. 

If ft receptors couple to K+ channels and adenylyl cyclase via different G proteins, it is possible that chronic morphine treatment uncouples the receptor from those G proteins linked to the K+ channel and not those coupling fi receptors to adenylyl cyclase. Such a hypothesis would require that G proteins couple to different intracellular domains of the fi receptor so that interaction of G proteins with some domains could be blocked by post-translational events, such as phosphorylation, whereas binding of G proteins to other fi receptor domains would not be affected. 

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