Conformational remodeling of proteasomal substrates by PA700, the 19 S regulatory complex of the 26 S proteasome.
PA700, the 19 S regulatory complex of the 26 S proteasome, plays a central role in the recognition and efficient degradation of misfolded proteins. PA700 promotes degradation by recruiting proteasomal substrates utilizing polyubiquitin chains and chaperone-like binding activities and by opening the access to the core of the 20 S proteasome to promote degradation. Here we provide evidence that PA700 in addition to binding misfolded protein substrates also acts to remodel their conformation prior to proteolysis. Scrambled RNase A (scRNase A), a misfolded protein, only slowly refolds spontaneously into an active form because of the rate-limiting unfolding of misfolded disulfide isomers. Notably, PA700 accelerates the rate of reactivation of scRNase A, consistent with its ability to increase the exposure of these disulfide bonds to the solvent. In this regard, PA700 also exposes otherwise buried sites to digestion by exogenous chymotrypsin in a polyubiquitinated enzymatically active substrate, pentaubiquitinated dihydrofolate reductase, Ub(5)DHFR. The dihydrofolate reductase ligand methotrexate counters the ability of PA700 to promote digestion by chymotrypsin. Together, these results indicate that in addition to increasing substrate affinity and opening the access channel to the catalytic sites, PA700 activates proteasomal degradation by remodeling the conformation of protein substrates.
Liu
Chang-Wei
CW
Department of Physiology, University of Texas Southwestern Medical Center at Dallas, Dallas, TX 75390, USA.
Millen
Linda
L
Roman
Tracie B
TB
Xiong
Hai
H
Gilbert
Hiram F
HF
Noiva
Robert
R
DeMartino
George N
GN
Thomas
Philip J
PJ
D000251:Adenosine Triphosphatases / Q000737:chemistry*; D000818:Animals; D020134:Catalytic Domain; D002417:Cattle; D002918:Chymotrypsin / Q000494:pharmacology; D004220:Disulfides / Q000737:chemistry; D004305:Dose-Response Relationship, Drug; D010450:Endopeptidases / Q000737:chemistry*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D008024:Ligands; D008727:Methotrexate / Q000494:pharmacology; D010100:Oxygen / Q000378:metabolism; D010455:Peptides / Q000737:chemistry; D046988:Proteasome Endopeptidase Complex; D011487:Protein Conformation; D017510:Protein Folding; D012259:Ribonuclease, Pancreatic / Q000737:chemistry; D013997:Time Factors
D004220:Disulfides; D004791:Enzyme Inhibitors; D008024:Ligands; D010455:Peptides; D012259:Ribonuclease, Pancreatic; D010450:Endopeptidases; D002918:Chymotrypsin; D046988:Proteasome Endopeptidase Complex; C119011:26S proteasome non-ATPase regulatory subunit 13; D000251:Adenosine Triphosphatases; D010100:Oxygen; D008727:Methotrexate