Assembly of an Evolutionarily Conserved Alternative Proteasome Isoform in Human Cells

Targeted intracellular protein degradation in eukaryotes is largelymediated by the proteasome. Here, we report the formation of an alternative proteasome isoform in human cells, previously found only in budding yeast, that bears an altered subunit arrangement in the outer ring of the proteasome core particle. These proteasomes result from incorporation of an additional alpha 4 (PSMA7) subunit in the position normally occupied by alpha 3 (PSMA4). Assembly of alpha 4-alpha 4'' proteasomes depends on the relative cellular levels of alpha 4 and alpha 3 and on the proteasome assembly chaperone PAC3. The oncogenic tyrosine kinases ABL and ARG and the tumor suppressor BRCA1 regulate cellular alpha 4 levels and formation of alpha 4-alpha 4 proteasomes. Cells primed to assemble alpha 4-alpha 4 proteasomes exhibit enhanced resistance to toxic metal ions. Taken together, our results establish the existence of an alternative mammalian proteasome isoform and suggest a potential role in enabling cells to adapt to environmental stresses.