Intracellular distribution and regulatory function of protein phosphatases

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Among the naturally occurring inhibitors of type 1 and type 2A protein phosphatases (PP1 and PP2A), tatutomycin (TM), first isolated from the bacterium Streptomyces spiroverticillatus, is unique in that it binds to and inhibits PP1 with higher affinity than it does PP2A . The ratio of the dissociation constant for the PP1-TM interaction to that for the PP2A-TM interaction (the PP1/PP2A ratio) is in the range 0.01-0.03. No specific structural factors responsible for its characteristic affinity to the enzymes have been identified until now.In the present experiments we have evaluated the contributions of the C1-C16 segment of TM to its binding to PP1 and PP2A.According to the current binding model, this relatively hydrophobic segment, which contains a spiroketal motif with two side chains, are supposed to be accommodated in a site which is slightly apart from the catalytic center of the enzymes. We therefore expected that chemical modifications of this segment might yield some derivative … More s retaining a measurable inhibitory activity. We were also particularly interested in the fact that the enantiomeric form of the spiroketal is present in the molecule of OA, which, in contrast to TM, exhibits exceedingly higher affinity to PP2A than to PP1. Since there are numerous examples of stereospecific interaction of a macromolecule with a relatively small ligand, it seemed natural to speculate that the stereochemistry of the spiroketal might be a key factor determining the affinity characteristics of the toxins.For these reasons we have chemically synthesized two TM analogues : TM1 in which the side chains of the spiroketal is removed but its stereochemistry is retained, and TM2 in which the spiroketal motif of TM1 is replaced with its enantiomeric form. We have observed that these derivatives indeed retain considerable inhibitory activities against PP1 and PP2A.The PP1/PP2A ratio, determined by dose-inhibition analyses using the native catalytic subunits of PP1 (PP1C) and PP2A and a recombinant γ isoform of PP1(PP1γ), is in the range 0.2-0.5 with TM1 and 5-10 with TM2. The marked increases in the ratio caused by the derivatization of TM to TM1 and TM2 indicate that the stereochemistry of the spiroketal as well as the presence of its side chains is an important factor for the characteristic affinities of TM to PP1 and PP2A. Less