Identical subunit topographies of human and yeast 20S proteasomes

The arrangement of subunits in human 20S proteasomes was recently determine d by us by immunoelectron microscopy and chemical cross-linking. The positi ons of 4 of the 14 subunits differed from those found in the yeast proteaso me by X-ray crystallography. Double labeling of human 20S proteasomes with antibodies to subunits C2 and C5 has now shown that these subunits are near est neighbors. The result contradicts our published model for the human pro teasome but is in accordance with the subunit arrangement in yeast proteaso mes, suggesting that yeast and human proteasomes most probably have identic al subunit arrangements. Immunoelectron microscopy also showed that the C-t erminal extension at the human C2 subunit is flexible but takes up a well-d efined position in the proteasome. (C) 1999 Academic Press.