Identification of the Pba1 and Pba2 Binding Sites on 20S Core Particle Intermediates

Abstract

The proteasome is responsible for breaking down the majority of the proteins in the cell. However, a complete understanding of how this large multi-subunit protease is assembled is currently lacking. Proper and timely assembly of the proteasome is critical for the functioning of the ubiquitin-proteasome pathway, defects in which have been associated with several different cancers. A recently discovered heterodimeric proteasome assembly chaperone, Pba1p-Pba2p, has been suggested to prevent the assembly process from straying off path. Pba1p-Pba2p associates with proteasomal assembly intermediates via C-terminal HbYX motifs. The HbYX motif is a tri-peptide sequence containing a hydrophobic residue (Hb) followed by a tyrosine (Y), then any amino acid (X). This motif was originally identified in proteasomal activators, and shown to mediate the association of activators with the proteasome by inserting into intersubunit pockets on either end of the proteasome. There are seven unique intersubunit binding pockets, located between neighboring α subunits on the proteasome, to which a HbYX-containing protein can bind; which of these pockets Pba1p-Pba2p binds to remains elusive. I attempted to identify where Pba1p and Pba2p bind via a crosslinking approach. Specific residues were mutagenized to cysteines on Pba1p, Pba2p, and the individual α subunits in order to generate crosslinkable species. By exposing yeast cells expressing these crosslinkable proteins to mild oxidizing conditions, I attempted to trap the Pba1p and Pba2p α intersubunit pocket interactions. In order to optimize crosslinking conditions, the assay was modified several ways. Additionally, measures were taken to increase detection of the crosslinked species via immunoblotting. Despite the efforts to improve the crosslinking and detection, I was unable to successfully detect a crosslinked species. However, crosslinking is a reasonable method to identify the Pba1p and Pba2p proteasomal binding sites, having been successfully used to identify binding sites for other HbYX-motif-containing proteins; further assay optimization should yield Pba1p and Pba2p proteasomal crosslinks.