PA28, an activator of the 20 S proteasome, is composed of two nonidentical but homologous subunits.

PA28, one of a series of a positive allosteric regulators of the 20 S proteasome, stimulates the enzyme's peptidase activities in an ATP-independent manner by binding to the terminal rings of the 20 S complex. PA28 has a native molecular mass of 180,000 Da and contains at least six subunits of approximately 28,000 Da. In this study we show that PA28 prepared from bovine heart contains two different subunits separable by reverse phase high performance liquid chromatography and that these subunits occur in approximately equal abundance. The subunits display mass values of 27,290 +/- 3.7 and 28,606 +/- 2.8 Da by electrospray mass spectrometry, showing that they differ in covalent structure. Partial amino acid sequence analysis of the subunits indicates that the subunits are the products of two different but homologous genes. A pair of subunits has also been isolated from rabbit heart, and partial amino acid sequence analysis shows each to be homologous to the corresponding subunit in bovine tissues. This indicates that the genes encoding two different polypeptide components of PA28 have been conserved during evolution and suggests the possibility that the two subunits play functionally distinct roles. Isolation of complexes formed between purified PA28 and the 20 S proteasome using density gradient centrifugation reveals that both PA28 subunits bind to the proteasome, indicating that both are components of functional PA28 molecules. These results are consistent with two alternative models for the subunit structure of PA28. There may exist two different PA28 molecules that are homooligomers of the 27,290- and 28,606-Da subunits, respectively. Alternatively, PA28 oligomers may contain mixtures of the 27,290- and 28,606-Da subunits either of fixed or variable stoichiometry.