alpha 5 subunit in Trypanosoma brucei proteasome can self-assemble to forma cylinder of four stacked heptamer rings

The proteasomes have a central role in catalysing protein degradation among both prokaryotes and eukaryotes. The 20 S proteasome constitutes their cat alytic core, In studying the structure of Trypanosoma brucei 20 S proteasom es, we isolated by two-dimensional (2D) gel electrophoresis a 27 kDa subuni t protein with an estimated pi of 4.7 and subjected it to mass spectrometri c analysis. A. tryptic peptide sequence from the protein was found identica l with that of the rat alpha 5 subunit. With the use of antiserum against T . brucei 20 S proteasomes to screen a T. b. rhodesiense lambda expression c DNA library, we obtained a cDNA clone encoding a full-length protein of 246 amino acid residues with a calculated molecular mass of 27 174 Da and a pi of 4.71. It bears 50.0 % and 46.3 % sequence identity with rat and yeast p roteasome subunit alpha 5 respectively, and matches all the peptide sequenc es derived from MS of the 2D gel-purified protein. The protein is thus desi gnated the alpha 5 subunit of T. brucei 20 S proteasome (TbPSA5). The recom binant protein, expressed in plasmid-transformed Escherichia coli, was foun d in a 27 kDa monomer form as well as polymerized forms with estimated mole cular masses ranging from 190 to 800 kDa. Under the electron microscope, th e most highly polymerized forms bear the appearance of cylinders of four-st acked heptamer rings with an estimated outer diameter of 14.5 nm and a leng th of 18 nm, which were immunoprecipitable by anti-(T. brucei 20S proteasom e) antiserum. In view of the documented self-assembly of the archaeon prote asome alpha subunit into double heptamer rings and the spontaneous assembly of the two alpha subunits from the 20 S proteasome of Rhodococcus erythrop olis, the self-assembly of the T. brucei alpha subunit might reflect a comm on feature of proteasome biogenesis shared by prokaryotes and primitive euk aryotes such as the trypanosomes but apparently lost among the higher forms of eukaryote such as the yeast and the mammals.