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The objective of this study was to identify the active form of the feline calicivirus (FCV) RNA-dependent RNA polymerase (RdRP). Multiple active forms of the FCV RdRP were identified. The most active enzyme was the full-length proteinase-polymerase (Pro-Pol) precursor protein, corresponding to amino acids 1072 to 1763 of the FCV polyprotein encoded by open reading frame 1 of the genome. Deletion of 163 amino acids from the amino terminus of Pro-Pol (the Val-1235 amino terminus) caused a threefold reduction in polymerase activity. Deletion of an additional one (the Thr-1236 amino terminus) or two (the Ala-1237 amino terminus) amino acids produced derivatives that were 7- and 175-fold, respectively, less active than Pro-Pol. FCV proteinase-dependent processing of Pro-Pol in the interdomain region preceding Val-1235 was not observed in th...
Okadaic acid (OA), dinophysistoxin-1 (DTX-1), and dinophysistoxin-2 (DTX-2) are algal toxins that can accumulate in shellfish and cause diarrhetic shellfish poisoning. Recent studies indicate that DTX-2 is about half as toxic and has about half the affinity for protein phosphatase 2A (PP2A) as OA. NMR structural studies showed that DTX-1 possessed an equatorial 35-methyl group but that DTX-2 had an axial 35-methyl group. Molecular modeling studies indicated that an axial 35-methyl could exhibit unfavorable interactions in the PP2A binding site, and this has been proposed as the reason for the reduced toxicity of DTX-2. Statistical analyses of published data indicate that the affinity of PP2A for DTX-1 is 1.6-fold higher, and for DTX-2 is 2-fold lower, than for OA. We obtained X-ray crystal structures of DTX-1 and DTX-2 bound to PP2A. T...
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