Distinct Elements in the Proteasomal β5 Subunit Propeptide Required for Autocatalytic Processing and Proteasome...

Area of Science:

• Cell biology
• Molecular biology
• Protein biochemistry

Background:

• Eukaryotic 20S proteasome assembly involves stacking subunits into four rings.
• Inner-ring subunits (β1, β2, β5) possess catalytic residues and N-terminal propeptides.
• Propeptides are autocatalytically removed late in assembly, a process not fully understood.

Purpose of the Study:

• To investigate the role of the β5 propeptide (β5pro) in Saccharomyces cerevisiae proteasome assembly.
• To elucidate the relationship between β5pro, the Ump1 assembly factor, and the β7 proteasome subunit.
• To clarify the timing of β7 incorporation and half-proteasome dimerization.

Main Methods:

• Genetic manipulation of the β5 propeptide in yeast.
• Analysis of proteasome maturation defects.
• Investigating interactions with Ump1 and β7 subunit.
• Studying conditions affecting half-proteasome dimerization.

Main Results:

• The N-terminal half of β5pro is dispensable, but sequences near the cleavage site are critical for assembly and self-cleavage.
• A conserved histidine residue promotes β5pro processing.
• Trans-expressed β5pro shows weak association with Ump1 precursors.
• β7 subunit incorporation precedes half-proteasome dimerization, with unequal roles for the β7 tail and β5pro in joining.

Conclusions:

• The β5 propeptide and β7 subunit play distinct roles in the late stages of proteasome assembly.
• β7 insertion into the half-mer occurs before dimerization, challenging previous assumptions.
• The β7 tail and β5pro contribute unequally to the crucial half-mer joining step.