Role of C-terminal Extensions of Subunits β2 and β7 in Assembly and Activity of Eukaryotic Proteasomes

Abstract

A close inspection of the crystal structure of the yeast 20 S proteasome revealed that a prominent connection between the two β-rings is mediated by the subunit β7/Pre4. Its C-terminal extension intercalates between the β1/Pre3 and β2/Pup1 subunits on the opposite ring. We show that the interactions promoted by the β7/Pre4 tail are important to facilitate the formation of 20 S particles from two half-proteasome precursor complexes and/or to stabilize mature 20 S proteasomes. The deletion of 19 residues from the β7/Pre4 C terminus leads to an accumulation of half-proteasome precursor complexes containing the maturation factor Ump1. The C-terminal extension of β7/Pre4, which forms several hydrogen bonds with β1/Pre3, is in addition required for the post-acidic activity mediated by the latter subunit. Deletion of the C-terminal tail of β7/Pre4 results in an inhibition of β1/Pre3 propeptide processing and abrogation of post-acidic activity. Our data obtained with yeast strains that expressed the mature form of Pre3 lacking its propeptide suggest that interactions between the Pre4 C terminus and Pre3 stabilize a conformation of its active site, which is essential for post-acidic activity. Deletion of the C-terminal extension of β2/Pup1, which wraps around β3/Pup3 within the same β-ring, is lethal, indicating that this extension serves an essential function in proteasome assembly or stability.