Redundancy of Mammalian Proteasome β Subunit Function during Endoplasmic Reticulum Associated Degradation

Publisher Website
Google Scholar
Abstract
Misfolded proteins in the endoplasmic reticulum (ER) are degraded by N-terminal threonine proteases within the 26S proteasome. Each protease is formed by an activated β subunit, β5/X, β1/Y, or β2/Z, that exhibits chymotrypsin-like, peptidylglutamyl-peptide hydrolyzing, or trypsin-like activity, respectively. Little is known about the relative contribution of specific β subunits in the degradation of endogenous protein substrates. Using active site proteasome inhibitors and a reconstituted degradation system, we now show that all three active β subunits can independently contribute to ER-associated degradation of the cystic fibrosis transmembrane conductance regulator (CFTR). Complete inactivation (>99.5%) of the β5/X subunit decreased the rate of ATP-dependent conversion of CFTR to trichloroacetic acid soluble fragments by only 40%. Similarly, proteasomes containing only active β1/Y or β2/Z subunits degraded CFTR at ∼50% of the rate observed for fully functional proteasomes. Simultaneous inhibition (>93%) of all three β subunits blocked CFTR degradation by ∼90%, and inhibition of both protease and ATPase activities was required to completely prevent generation of small peptide fragments. Our results demonstrate both a conserved hierarchy (ChT-L > PGPH ≥ T-L) as well as a redundancy of β subunit function and provide insight into the mechanism by which active site proteasome inhibitors influence degradation of endogenous protein substrates at the ER membrane.

This publication has 14 references indexed in Scilit: