Regulatory Mechanism Of Phosphatase UBLCP1 In Proteasome Assembly And Function

The Ubiquitin-Proteasome System(UPS)is responsible for the degradation of majority proteins and plays an essential role in maintaining proteostasis and cellular functions.With the help of ubiquitin-activating enzyme,ubiquitin-conjugating enzyme and ubiquitin ligase,ubiquitin sequentially binds to target proteins with covalent linkage.Polyubiquitinated proteins will be escorted to proteasome for recognition and degradation.The 26S proteasome is a huge proteasome complex,with over 32 different proteasome subunits.Aberrant proteasome assembly or impaired proteasome activity will lead to aberrant proteostasis and aberrant cellular functions.Ubiquitin-like domain-containing C-terminal domain phosphatase 1(UBLCP1),a FCP/SCP phosphatase family member,was identified as the first proteasome phosphatase.UBLCP1 binds to proteasome subunit Rpnl and dephosphorylates proteasome in vitro.However,it is still unclear which proteasome subunit(s)are the bona fide substrate(s)of UBLCP1 and thus the precise mechanism underlying its role in regulating proteasome remains elusive.We show that UBLCP1 selectively binds to the 19S regulatory particle(RP)through its interaction with Rpnl,but not the 20S core particle(CP)or the 26S proteasome holoenzyme.In the RP,UBLCP1 dephosphorylates the subunit Rptl,impairs its ATPase activity,and consequently disrupts the 26S proteasome assembly,yet it has no effects on the RP assembly from precursor complexes.UBLCP1 significantly impair proteasome activity and stabilize proteasome substrates.Both the Rpnl-binding and phosphatase activities of UBLCP1 are essential for its function on Rptl dephosphorylation and regulating proteasome activity in vivo and in vitro.Thus,our study delineates the detailed mechanism underlying phosphatase UBLCP1 in regulating proteasome activity.Through binding to Rpn1,UBLCP1 interacts with and dephosphorylates proteasome subunits Rptl.UBLCP1 impair the ATPase activity of Rptl and proteasome assembly,which directly leads to impaired proteasome activity.