Protein phosphorylation
[10] In 1906, Phoebus Levene at the Rockefeller Institute for Medical Research identified phosphate in the protein vitellin (phosvitin)[9] and by 1933 had detected phosphoserine in casein, with Fritz Lipmann.
Earl Sutherland explained in 1950, that the activity of phosphorylase was increased and thus glycogenolysis stimulated when liver slices were incubated with adrenalin and glucagon.
In 1975, it was shown that cAMP-dependent proteins kinases phosphorylate serine residues on specific amino acid sequence motifs.
[13] In the early 1980, the amino-acid sequence of the first protein kinase was determined which helped geneticists understand the functions of regulatory genes.
[14][15] Edmond Fischer and Edwin Krebs were awarded the Nobel prize in 1992 "for their discoveries concerning reversible protein phosphorylation as a biological regulatory mechanism".
Because of the ease with which proteins can be phosphorylated and dephosphorylated, this type of modification is a flexible mechanism for cells to respond to external signals and environmental conditions.
Reversible phosphorylation results in a conformational change in the structure in many enzymes and receptors, causing them to become activated or deactivated.
[26] In prokaryotic proteins phosphorylation occurs on the serine, threonine, tyrosine, histidine, arginine or lysine residues.
[30][citation needed] This is the mechanism in many forms of signal transduction, for example the way in which incoming light is processed in the light-sensitive cells of the retina.
[39] They have been successfully used to identify dynamic changes in the phosphorylation status of more than 6,000 sites after stimulation with epidermal growth factor.
However, it has a negative impact on several fundamental biological processes such as transcription, replication and DNA repair by restricting the accessibility of certain enzymes and proteins.
Researchers choose proteins that are known to modify histones to test their effects on transcription, and found that the stress-induced kinase, MSK1, inhibits RNA synthesis.
The analysis of phosphorylated histidine using standard biochemical and mass spectrometric approaches is much more challenging than that of Ser, Thr or Tyr.
[49][7][5] and[50] In prokaryotes, archaea, and some lower eukaryotes, histidine's nitrogen act as a nucleophile and binds to a phosphate group.
While tyrosine phosphorylation is found in relatively low abundance, it is well studied due to the ease of purification of phosphotyrosine using antibodies.
CDK inhibitors (CKIs) block kinase activity in the cyclin-CDK complex to halt the cell cycle in G1 or in response to environmental signals or DNA damage.
This enables proteins to stay inbound within a cell since the negative phosphorylated site disallows their permeability through the cellular membrane.
A curated database of dbPAF was created, containing known phosphorylation sites in H. sapiens, M. musculus, R. norvegicus, D. melanogaster, C. elegans, S. pombe and S. cerevisiae.
[57] Other tools of phosphorylation prediction in proteins include NetPhos[58] for eukaryotes, NetPhosBac[58] for bacteria, and ViralPhos[59] for viruses.
Cell growth, differentiation, migration, and metabolic homeostasis are cellular processes maintained by tyrosine phosphorylation.
Indeed, phosphorylation replaces neutral hydroxyl groups on serines, threonines, or tyrosines with negatively charged phosphates with pKs near 1.2 and 6.5.
In some very specific cases, the detection of the phosphorylation as a shift in the protein's electrophoretic mobility is possible on simple 1-dimensional SDS-PAGE gels, as it is described for instance for a transcriptional coactivator by Kovacs et al.[67] Strong phosphorylation-related conformational changes (that persist in detergent-containing solutions) are thought to underlie this phenomenon.
Compared to the usual fragmentation methods, EThcD scheme provides more informative MS/MS spectra for unambiguous phosphosite localization.
[73][74] Protein phosphorylation is common among all clades of life, including all animals, plants, fungi, bacteria, and archaea.
Targets of CDK phosphorylation often have phosphosites in disordered segments, which are found in non-identical locations even in close species.
While CDK activity is critical for cell growth and survival in all eukaryotes, only very few phosphosites show strong conservation of their precise positions.
It is known that eukaryotes rely on the phosphorylation of the hydroxyl group on the side chains of serine, threonine, and tyrosine for cell signaling.
These are the main regulatory post-translational modifications in eukaryotic cells but the protein phosphorylation of prokaryotes are less intensely studied.
[82] Phosphatases PP1, PP2A, PP2B, and PP2C dephosphorylate tau protein in vitro, and their activities are reduced in areas of the brain in Alzheimer patients.
Alzheimer disease tau seems to remove MAP1 and MAP2 (two other major associated proteins) from microtubules and this deleterious effect is reversed when dephosphorylation is performed, evidencing hyperphosphorylation as the sole cause of the crippling activity.
