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Nat Struct Mol Biol. 2008 Mar;15(3):237-44. doi: 10.1038/nsmb.1389. Epub 2008 Feb 17.

A multimeric assembly factor controls the formation of alternative 20S proteasomes.

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Department of Molecular Biophysics & Biochemistry, Yale University, 266 Whitney Avenue, New Haven, Connecticut 06520-8114, USA.


The proteasome is the central regulatory protease of eukaryotic cells. Heteroheptameric alpha-subunit and beta-subunit rings stack to form the 20S proteasome, which associates with a 19S regulatory particle (RP). Here we show that two yeast proteins, Pba3 and Pba4, form a previously unidentified 20S proteasome-assembly chaperone. Pba3-Pba4 interacts genetically and physically with specific proteasomal alpha subunits, and loss of Pba3-Pba4 causes both a reduction and a remodeling of cellular proteasomes. Notably, mutant cells accumulate proteasomes in which a second copy of the alpha4 subunit replaces alpha3. 20S proteasome-assembly defects also are associated with altered RP assembly; this unexpected result suggests that the 20S proteasome can function as an RP-assembly factor in vivo. Our data demonstrate that Pba3-Pba4 orchestrates formation of a specific type of proteasome, the first example of a trans-acting factor that controls assembly of alternative proteasomal complexes.

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