Molecular cloning of cDNAs for two Xenopus proteasome subunits and their expression in adult tissues

Proteasome, a large protein complex with ATP-dependent protease activities, is composed of non-identical but closely related multi-subunits. Using cDNAs for rat proteasome subunits as probes, we obtained three cDNA clones for two Xenopus proteasome subunits from ovary cDNA library. The primary structures of the three cDNAs showed high homology to the corresponding proteasome subunits of other mammalian species (above 90%) and also considerable homology to those of Drosophila and yeast. These results indicate that the sequences of proteasome subunits are well conserved during evolution. Northern blot hybridization revealed that RNAs for the newly isolated subunits (XC8 and XC9) and the previously isolated subunit (XC3) occur at very high levels in testis and ovary, at moderately high levels in lung, skin kidney and spleen, and at low levels in liver, stomach and muscle. It was also shown that relative amounts of the mRNAs for the three subunits are similar in all the adult tissues examined. From these results, we concluded that the expression of the genes for the three subunits (XC3 XC8 and XC9-1) takes place in a roughly coordinated manner in different adult tissues.