Activation of transfer RNA-guanine ribosyltransferase by protein kinase C

摘要

Transfer RNA-guanine ribosyltransferase (TGRase) irreversibly incorporates queulne into the first position in the anticodon of four tRNA isoacceptors. Rat brain protein kinase C (PKC) was shown to stimulate rat liver TGRase activity. TGRase preparations derived from rat liver have been observed to decrease in activity over time in storage at -20 or -70 °C. Contamination of the samples by phosphatases was indicated by a p-nitro-phenylphosphate conversion test. The addition of micromolar concentrations of the phosphatase inhibitors sodium pyrophosphate and sodium fluoride into TGRase isolation buffers resulted in a greater return of TGRase activity than without these inhibitors. Inactive TGRase preparations were reactivated to their original activitywith the addition of PKC. In assays combining both TGRase and PKC enzymes, inhibitors of protein kinase C (sphingosine, staurosporine, H-7 and caiphostin C) all blocked the reactivation of TGRase, whereas activators of protein kinase C (calcium, diacylglycerol and phosphatidyl serine) increased the activity of TGRase. None of the PKC modulators affected TGRase activity directly. Alkaline phosphatase, when added to assays, decreased the activity of TGRase and also blocked the reactivation of TGRase withPKC. Denaturing PAGE and autoradiography was performed on TGRase isolates that had been labelled with ~(32)P by PKC. The resulting strong 60 kDa band (containing the major site for phosphorylation) and weak 34.5 kDa band (containing the TGRase activity)are suggested to associate to make up a 104 kDa heterodimer that comprises the TGRase enzyme. This was corroborated by native and denaturing size-exclusion chromatography. These results suggest that PKC-dependent phosphorylation of TGRase is tied to efficient enzymatic function and therefore control of the queuine modification of tRNA.