MOLECULAR-ORGANIZATION OF THE 20S PROTEASOME GENE FAMILY FROM ARABIDOPSIS-THALIANA

The 20S proteasome is the proteolytic complex in eukaryotes responsibl e for degrading short-lived and abnormal intracellular proteins, espec ially those targeted by ubiquitin conjugation. The 700-kD complex exis ts as a hollow cylinder comprising four stacked rings with the catalyt ic sites located in the lumen. The two outer rings and the two inner r ings are composed of seven different alpha and beta polypeptides, resp ectively, giving an alpha 7/beta 7/beta 7/alpha 7 symmetric organizati on. Here we describe the molecular organization of the 20S proteasome from the plant Arabidopsis thaliana. From an analysis of a collection of cDNA and genomic clones, we identified a superfamily of 23 genes en coding all 14 of the Arabidopsis proteasome subunits, designated PAA-P AG and PBA-PBG for Proteasome Alpha and Beta subunits A-G, respectivel y. Four of the subunits likely are encoded by single genes, and the re maining subunits are encoded by families of at least 2 genes. Expressi on of the alpha and beta subunit genes appears to be coordinately regu lated. Three of the nine Arabidopsis proteasome subunit genes tested, PAC1 (alpha 3), PAE1 (alpha 5) and PBC2 (beta 3), could functionally r eplace their yeast orthologs, providing the first evidence for cross-s pecies complementation of 20S subunit genes. Taken together, these res ults demonstrate that the 20S proteasome is structurally and functiona lly conserved among eukaryotes and suggest that the subunit arrangemen t of the Arabidopsis 20S proteasome is similar if not identical to tha t recently determined for the yeast complex.