Dephosphorylation, regulation

Protein phosphorylation dephosphorylation regulates the activity of enzymic and nonenzymic proteins in eukaryotic cells. Ten per cent or more of all proteins in a cell are modified in that way. The phosphates are transferred from ATP and esterified with hydroxyl groups of serine, threonine, or tyrosine residues. They are removed and transferred to water by phosphatases. There are at least about 2000 kinases and about 1000 phosphatases to carry out these reactions (Fig. 7.1). ... 

Cyclic Protein Phosphorylation and Dephosphorylation Regulate Many Cellular Functions... 

Phosphorylation and dephosphorylation regulate the ion channels in guard cells in response to ABA... 

Canaani, O., Barber, J., and Malkin, S. (1984). Evidence that phosphorylation and dephosphorylation regulate the distribution of exci-... 

The last part of this account will be devoted to protein kinases and protein phosphatases and some recent results we have obtained for them. Protein kinases and phosphatases are signaling biomolecules that control the level of phosphorylation and dephosphorylation of tyrosine, serine or threonine residues in other proteins, and by this means regulate a variety of fundamental cellular processes including cell growth and proliferation, cell cycle and cytoskeletal integrity. 

STAT binding site in the promoter region of cytokine-responsive genes. It is nonameric palindrome with relaxed sequence specificity.) sites, to regulate gene expression. Tyrosine phosphatases located in the nucleus then dephosphorylate STAT molecules (Fig. 1). 

Calcium-dependent regulation involves the calcium-calmodulin complex that activates smooth muscle MLCK, a monomer of approximately 135 kDa. Dephosphorylation is initiated by MLCP. MLCP is a complex of three proteins a 110-130 kDa myosin phosphatase targeting and regulatory subunit (MYPT1), a 37 kDa catalytic subunit (PP-1C) and a 20 kDa subunit of unknown function. In most cases, calcium-independent regulation of smooth muscle tone is achieved by inhibition of MLCP activity at constant calcium level inducing an increase in phospho-rMLC and contraction (Fig. 1). 

The major relaxing transmitters are those that elevate the cAMP or cGMP concentration (Fig. 3). Adenosine stimulates the activity of cAMP kinase. The next step is not clear, but evidence has been accumulated that cAMP kinase decreases the calcium sensitivity of the contractile machinery. In vitro, cAMP kinase phosphorylated MLCK and decreased thereby the affinity of MLCK for calcium-calmodulin. However, this regulation does not occur in intact smooth muscle. Possible other substrate candidates for cAMP kinase are the heat stable protein HSP 20, (A heat stable protein of 20 kDa that is phosphorylated by cGMP kinase. It has been postulated that phospho-HSP 20 interferes with the interaction between actin and myosin allowing thereby smooth muscle relaxation without dephosphorylation of the rMLC.) Rho A and MLCP that are phosphorylated also by cGMP kinase I (Fig. 3). 

For the purpose of discussion, crossbridge regulation can be split into three overlapping sets of reactions (a) the Ca-calmodulin cascade (MLCK activation), (b) the phosphorylation-dephosphorylation cycle (the Four State Model), and (c) actin-myosin cycle (chemomechanical transduction). 

Phosphorylation-dephosphorylation. The site on myosin which is phosphory-lated is not the same as the site by which it attaches to actin. Therefore, there are two geometrically separate reactions in regulation and from the Law of Reversibility there must be at least some myosin molecules in at least four different states ... 

Figure 9-7. Covalent modification of a regulated enzyme by phosphorylation-dephosphorylation of a seryl residue.

Protein phosphorylation-dephosphorylation is a highly versatile and selective process. Not all proteins are subject to phosphorylation, and of the many hydroxyl groups on a protein s surface, only one or a small subset are targeted. While the most common enzyme function affected is the protein s catalytic efficiency, phosphorylation can also alter the affinity for substrates, location within the cell, or responsiveness to regulation by allosteric ligands. Phosphorylation can increase an enzyme s catalytic efficiency, converting it to its active form in one protein, while phosphorylation of another converts it into an intrinsically inefficient, or inactive, form (Table 9—1). 

Many proteins can be phosphorylated at multiple sites or are subject to regulation both by phosphorylation-dephosphorylation and by the binding of allosteric ligands. Phosphorylation-dephosphorylation at any one site can be catalyzed by multiple protein kinases or protein phosphatases. Many protein kinases and most protein phosphatases act on more than one protein and are themselves interconverted between active and inactive forms by the binding of second messengers or by covalent modification by phosphorylation-dephosphorylation. 

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