ATP binding by proteasomal ATPases regulates cellular assembly and substrate-induced functions of the 26 S proteasome

Area of Science:

• Cell Biology
• Molecular Biology
• Biochemistry

Background:

• The 26S proteasome is a critical cellular machine responsible for protein degradation.
• Its function relies on the ATPase subunits (Rpt1-6) that regulate its activity.
• Understanding the specific roles of each subunit is key to comprehending proteasome regulation.

Purpose of the Study:

• To investigate the distinct roles of ATP binding by individual Rpt subunits in proteasome assembly and function.
• To determine how ATP binding mutations in Rpt subunits affect the 26S proteasome's molecular activities.
• To elucidate the mechanisms by which substrates stimulate proteasome function.

Main Methods:

• Site-directed mutagenesis was used to create ATP binding-deficient mutants of Rpt1-6 subunits.
• Proteasome assembly was assessed in intact cells and in vitro using assembly assays.
• ATPase activity, substrate gate opening, and protein degradation assays were performed on purified proteasomes.

Main Results:

• Mutations in Rpt1-4 prevented proteasome assembly, while Rpt5/Rpt6 mutations did not affect assembly.
• Purified proteasomes with mutant Rpt5 or Rpt6 showed normal basal ATPase activity but were defective in ATP-dependent protein degradation.
• Substrate-stimulated ATPase and peptidase activities were abolished in proteasomes with mutant Rpt5 or Rpt6.

Conclusions:

• ATP binding by Rpt1-4 is essential for 26S proteasome assembly.
• ATP binding by Rpt5 and Rpt6 is critical for substrate-dependent proteasome functions, including degradation.
• Substrate-stimulated activity and gate opening require the coordinated action of all ATP-binding-competent Rpt subunits, highlighting their importance in regulating proteolysis.