Subunit stoichiometry of human proteasomes.

Subunits from human placental proteasomes were separated by two-dimensional polyacrylamide gel electrophoresis. The amino acid composition of proteins from individual spots were determined. Some of the spots had identical amino acid compositions, confirming that they contain isoforms of the same subunit. Proteasomes from HeLa cells, labelled with 3H-leucine, were precipitated with an antibody and similarly separated into subunits. The radioactivity in each subunit was measured. The subunit stoichiometry was then calculated from these data and the leucine contents in the subunits. Each of the 14 major subunits of human proteasomes are apparently present in equal amounts.