PubMed · 5725220
The source did not provide an abstract. Follow the original record for more information.
Explore connections, maps & timelines
C J Pedersen. Ionic complexes of macrocyclic polyethers.. https://pubmed.ncbi.nlm.nih.gov/5725220/
Cite the original work for its findings. Save a collection to share your selection of sources.
KEEP EXPLORING
The repetitive C-terminal domain (CTD) of RNA polymerase (RNAP) II is extensively phosphorylated concomitant with the initiation of transcription and must be dephosphorylated before RNAP II can begin another round of transcription. A CTD phosphatase was purified more than 7,500-fold from a HeLa cell extract. SDS-polyacrylamide gel electrophoresis shows a predominant protein of 205 kDa and a less abundant protein of 150 kDa co-eluting with the CTD phosphatase activity. Sedimentation and gel filtration analysis suggest that CTD phosphatase has an elongated structure with a M(r) of 200,000. This enzyme is a type 2C phosphatase in that it requires Mg2+ for activity and is resistant to okadaic acid. CTD phosphatase appears to processively dephosphorylate the CTD and is specific in that it does not dephosphorylate phosphorylase a, the alpha or beta subunits of phosphorylase kinase or RNAP II phosphorylated with casein kinase II. CTD phosphatase dephosphorylates RNAP IIO purified from calf thymus or generated in vitro by two previously described CTD kinases. These results suggest that CTD phosphatase has the properties expected for a protein phosphatase that catalyzes the conversion of RNAP IIO to RNAP IIA and may play a key role in the transcription cycle of RNAP II.
Okadaic acid (OA), a potent inhibitor of type-1 and type-2A protein phosphatases (PP1 and PP2A), has four hydroxyl groups at 2, 7, 24 and 27 positions (see Figure 1). By chemical treatment of OA we synthesized a derivative, in which the 27-hydroxyl group was specifically oxidized (27-dehydro-OA). The inhibitory effect of this OA derivative was examined on the activities of PP1 and PP2A, which were inhibited by intact OA with dissociation constants (Ki) of 150 nM and 32 pM respectively. We found that the affinity of OA was decreased 40-fold (Ki = 6 microM) with PP1 and 230-fold (Ki = 7.3 nM) with PP2A after oxidation of the 27-hydroxyl group. According to the model of the three-dimensional conformation of OA on the basis of X-ray analyses, the 27-hydroxyl group appears to be present in a position relatively free from intramolecular bonding formation, in comparison with the other three hydroxyl groups. The marked increases in the Ki values for PP1 and PP2A, which indicate the reduction of the absolute values of the free energy of binding by 9 kJ/mol and 14 kJ/mol respectively, may imply that the 27-hydroxyl group serves as a binding site with the phosphatase molecules.
Okadaic acid, a cell-permeant inhibitor of serine/threonine phosphatases (PP) was found to attenuate the IgE-mediated release of both performed (histamine) and newly formed (leukotriene C4) mediators from human lung mast cells (HLMC). Optimal inhibition (82 +/- 6%) of the IgE-triggered release of histamine was observed after a 2-h incubation of HLMC with okadaic acid (1 microM). Shorter (< 2 h) incubation times and lower (< 1 microM) concentrations of okadaic acid were less effective at inhibiting histamine release from HLMC. The extent to which okadaic acid prevented histamine release was highly dependent upon the strength of the stimulus with lower levels of stimulation more readily modulated. The efficacy of okadaic acid as an inhibitor was not dependent upon the nature of the stimulus because matched levels of histamine release (approximately 30%) induced by either IgE-mediated processes or the calcium ionophore A23187 were attenuated to comparable extents. A series of analogues of okadaic acid and a structurally-distinct PP inhibitor, calyculin, were also found to inhibit the IgE-induced release of mediators from HLMC with the following rank order of potency; calyculin (approximate IC50; 0.015 microM) > okadaic acid (0.2 microM) > okadaol (3.3 microM) > nor-okadaone (> 10 microM). These same PP inhibitors displayed a similar rank order of activity for the inhibition of mediator release induced by the ionophore A23187. Extracts of purified HLMC were found to liberate 32P from radiolabeled glycogen phosphorylase and this PP activity was inhibited by both low (2 nM) and high (5 microM) concentrations of okadaic acid suggesting that HLMC contain both PP type 2A (PP2A) and PP type 1 (PP1). The okadaic acid, analogues of okadaic acid, and calyculin attenuated mediator release at concentrations and with a rank order of activity that closely parallels their activities as inhibitors of PP suggests that PP are important in the regulation of HLMC function. This contention is further supported by the finding that PP activities are constitutively associated with HLMC and that these activities could be inhibited by okadaic acid.