Critical elements in proteasome assembly

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Abstract
Coexpression of both subunits of the Thermoplasma proteasome in Escherichia coli yields fully assembled and proteolytically active proteasomes. Post-translational processing of the β-subunit occurs in E. coli as it does in Thermoplasma. Coexpression of the α-subunit and the βΔpro-subunit, a mutant β-subunit lacking the propeptide, also yields fully assembled and active proteasomes. This indicates that the β-propeptide is not essential for the folding and assembly of Thermoplasma proteasomes. Separately expressed α-subunits assemble into heptameric rings indistinguishable from the terminal rings of a proteasome. Mutational analysis shows that the amino terminus, which is highly conserved in all proteasomal α-type proteins, is essential for assembly. In the absence of α-subunits the β-subunits are monomeric and post-translational processing of the β-propeptide does not occur.