Given the recent finding by Winklhofers group that parkin is capable of mediating linear ubiquitin chain assembly [25], there is a possibility that the K0 mutant could support linear ubiquitination of synphilin-1 in the presence of parkin thereby leading to the stabilization of the protein. and reprobed with anti-actin antibody to reflect loading variations. These experiments were duplicated with similar results. (B) Bar graph showing the chymotrypsin-like proteasome activities of lysates prepared from untreated cells or those treated with various proteasome inhibitors, as indicated (* 0.05, ** 0.001 vs. column 1, Students 0.05, ** 0.001, Students 3). Amplified product was digested with EcoRV and MunI and inserted into EcoRV and EcoRI sites of pL6mCWmIRESCherry. The lentivector pL6mCWmIRESCherry was modified from pLenti6/V5-D-TOPO (Invitrogen) Nuciferine by reengineering of the multiple cloning site, insertion of the cPPT and WPRE elements, and insertion of the IRESmCherry reporter cassette. Lentivirus packaging was performed in 293FT cells according to the protocol provided with the ViraPower? Lentiviral Directional TOPO? Expression Kit (Invitrogen). Lentivirus particles were concentrated from cell culture supernatant according to the protocol of Deiseroth Lab (http://www.stanford.edu/group/dlab/resources/lvprotocol.pdf). Lentivirus carrying the ubiquitin expression constructs was used to transduce wild type or Ubc13 knockout MEFs. Prior to transduction, cells were cultured to ~90% confluence. Concentrated virus particles were added to cell culture medium containing 6 g/ml of Polybrene. Long term transgene expression was maintained by selecting for resistance to Blasticidin S at a final concentration of 10 g/ml. Transgene expression was detected by mCherry epifluorescence. Inclusion formation and autophagic removal The autophagic clearance of Nuciferine inclusions formed under conditions of proteasomal impairment was investigated using a method originally described by Fortun et al [22]. Cells were first treated with 5 M lactacystin to facilitate inclusion formation. After 16 h incubation, the treated cells were washed out and allowed to recover in normal media for 24 h. Concurrently, a parallel set of similarly treated cells were incubated with starvation media (1% serum) to stimulate autophagy. Thereafter, cells were processed for immunocytochemical staining for blinded evaluation of inclusions. Statistical significance for all the quantitative data obtained was analyzed using Students 0.05, ** 0.001) unless otherwise stated. Results K63-polyubiquitination is enhanced in parkin-expressing cells in the presence of proteasome inhibition Recently, K63-specific antibodies have become available from commercial sources. Although we have independently confirmed its linkage specificity in the present study (Figure S1A), we found that the sensitivity of commercially available K63 antibodies towards endogenously promoted K63 linkages under normal cell culture conditions (i.e. in the absence of proteasome inhibition) to be rather weak (not shown). To circumvent this problem, we performed our subsequent Nuciferine experiments in cells expressing exogenous HA-tagged wild type ubiquitin. Notably, we observed that exogenously-introduced K63 ubiquitin species (as visualized via anti-UbK63 staining) tend to reside in the pellet fraction of cell lysate (Figure S1B & C), which is consistent with our previous finding that K63-ubiquitination could influence the cellular distribution of proteins [6]. To test our hypothesis that parkin-mediated K63 ubiquitination may be enhanced in cells undergoing proteasomal stress, we next examined the immunoreactivity of anti-UbK63 in Triton-X-100-soluble (S) and -insoluble (P) lysates sequentially prepared from parkin-expressing cells in the presence or absence of proteasome inhibition. We detected a modest but significant increase in the Nuciferine levels of K63-linked polyubiquitination specifically in the P fraction in untreated cells expressing parkin compared to control cells (Figure 1A). Importantly, when these parkin-expressing cells were treated with the proteasome inhibitor, MG132, we observed a dramatic increase in the level of anti-UbK63 immunoreactivity, which again resides predominantly in the P fraction (Figure 1A). The same phenomenon is observed when parkin-expressing cells were treated with PSI and lactacystin, two other proteasome inhibitors but not with DMSO vehicle (Figure S2A-B). Substituting parkin with a truncation mutant deleted of its C-terminal catalytic RING domain (RING) significantly reduces the level of K63 polyubiquitinated proteins in cells treated with MG132, as are substitutions Rabbit polyclonal to Vang-like protein 1 with disease-associated RING mutants, T240R, T415N and G430D (Figure 1B). On the other hand, a parkin mutant carrying the M192L mutation, which resides outside the RING catalytic domain, retains the ability to promote K63-linked polyubiquitination (Figure 1B). Our results thus suggest that proteasome inhibition promotes parkin-mediated K63-linked ubiquitination, an activity that is clearly dependent on the integrity of its RING domain. To extend this finding, we also repeated our experiment with MG132-treated cells expressing other E3 members. Anti-UbK63 immunoblotting of lysates prepared from these variously transfected cells revealed that Siah-1, for which no association with K63-linked polyubiquitination has been reported to date, as well as two other RING-containing E3s, HHARI and Cbl, failed to enhance the levels of K63.
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