The 20S proteasome is an essential, 28-subunit protease that sequesters proteolytic sites within a central chamber, thereby
repressing substrate degradation until proteasome activators open the entrance/exit gate. Two established activators, Blm10
and PAN/19S, induce gate opening by binding to the pockets between proteasome α-subunits using C-terminal HbYX (hydrophobic-tyrosine-any residue) motifs. Equivalent HbYX motifs have been identified in Pba1 and Pba2, which function in proteasome assembly. Here, we demonstrate that Pba1-Pba2
proteins form a stable heterodimer that utilizes its HbYX motifs to bind mature 20S proteasomes in vitro and that the Pba1-Pba2 HbYX motifs are important for a physiological function of proteasomes, the maintenance of mitochondrial function. Other factors
that contribute to proteasome assembly or function also act in the maintenance of mitochondrial function and display complex
genetic interactions with one another, possibly revealing an unexpected pathway of mitochondrial regulation involving the
Pba1-Pba2 proteasome interaction. Our determination of a proteasome Pba1-Pba2 crystal structure reveals a Pba1 HbYX interaction that is superimposable with those of known activators, a Pba2 HbYX interaction that is different from those reported previously, and a gate structure that is disrupted but not sufficiently
open to allow entry of even small peptides. These findings extend understanding of proteasome interactions with HbYX motifs and suggest multiple roles for Pba1-Pba2 interactions throughout proteasome assembly and function.
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"While this manuscript was in review process, a paper appeared in press describing a structural study of yeast proteasome assembly chaperones Pba1/Pba2 . Pba1 and Pba2 formed a heterodimer, although the overall structures were similar to that of PbaB. "
[Show abstract][Hide abstract]ABSTRACT: Assembly of the eukaryotic 20S proteasome is an ordered process involving several proteins operating as proteasome assembly factors including PAC1-PAC2 but archaeal 20S proteasome subunits can spontaneously assemble into an active cylindrical architecture. Recent bioinformatic analysis identified archaeal PAC1-PAC2 homologs PbaA and PbaB. However, it remains unclear whether such assembly factor-like proteins play an indispensable role in orchestration of proteasome subunits in archaea. We revealed that PbaB forms a homotetramer and exerts a dual function as an ATP-independent proteasome activator and a molecular chaperone through its tentacle-like C-terminal segments. Our findings provide insights into molecular evolution relationships between proteasome activators and assembly factors.
[Show abstract][Hide abstract]ABSTRACT: The proteasome refers to a collection of complexes centered on the 20S proteasome core particle (20S CP), a complex of 28 subunits that houses proteolytic sites in its hollow interior. Proteasomes are found in eukaryotes, archaea, and some eubacteria, and their activity is critical for many cellular pathways. Important recent advances include inhibitor binding studies and the structure of the immunoproteasome, whose specificity is altered by the incorporation of inducible catalytic subunits. The inherent repression of the 20S CP is relieved by the ATP-independent activators 11S and Blm10/PA200, whose structures reveal principles of proteasome mechanism. The structure of the ATP-dependent 19S regulatory particle, which mediates degradation of polyubiquitylated proteins, is being revealed by a combination of crystal or NMR structures of individual subunits and electron microscopy reconstruction of the intact complex. Other recent structural advances inform us about mechanisms of assembly and the role of conformational changes in the functional cycle. Expected final online publication date for the Annual Review of Biophysics Volume 42 is May 06, 2013. Please see http://www.annualreviews.org/catalog/pubdates.aspx for revised estimates.
No preview · Article · Feb 2013 · Annual Review of Biophysics
[Show abstract][Hide abstract]ABSTRACT: The eukaryotic ubiquitin-proteasome system is responsible for most aspects of regulatory and quality-control protein degradation in cells. Its substrates, which are usually modified by polymers of ubiquitin, are ultimately degraded by the 26S proteasome. This 2.6-MDa protein complex is separated into a barrel-shaped proteolytic 20S core particle (CP) of 28 subunits capped on one or both ends by a 19S regulatory particle (RP) comprising at least 19 subunits. The RP coordinates substrate recognition, removal of substrate polyubiquitin chains, and substrate unfolding and translocation into the CP for degradation. Although many atomic structures of the CP have been determined, the RP has resisted high-resolution analysis. Recently, however, a combination of cryo-electron microscopy, biochemical analysis, and crystal structure determination of several RP subunits has yielded a near-atomic-resolution view of much of the complex. Major new insights into chaperone-assisted proteasome assembly have also recently emerged. Here we review these novel findings. Expected final online publication date for the Annual Review of Biochemistry Volume 82 is June 02, 2013. Please see http://www.annualreviews.org/catalog/pubdates.aspx for revised estimates.
Full-text · Article · Mar 2013 · Annual review of biochemistry