Key points are not available for this paper at this time.
The peroxisomal protein import receptor Pex5p is modified by ubiquitin, both in an Ubc4p-dependent and -independent manner. Here we show that the two types of ubiquitination target different residues in the NH2-terminal region of Pex5p and we identify Pex4p (Ubc10p) as the ubiquitin-conjugating enzyme required for Ubc4p-independent ubiquitination. Whereas Ubc4p-dependent ubiquitination occurs on two lysine residues, Pex4p-dependent ubiquitination neither requires lysine residues nor the NH2-terminal α-NH2 group. Instead, a conserved cysteine residue appears to be essential for both the Pex4p-dependent ubiquitination and the overall function of Pex5p. In addition, we show that this form of ubiquitinated Pex5p is susceptible to the reducing agent β-mercaptoethanol, a compound that is unable to break ubiquitin-NH2 group linkages. Together, our results strongly suggest that Pex4p-dependent ubiquitination of Pex5p occurs on a cysteine residue. The peroxisomal protein import receptor Pex5p is modified by ubiquitin, both in an Ubc4p-dependent and -independent manner. Here we show that the two types of ubiquitination target different residues in the NH2-terminal region of Pex5p and we identify Pex4p (Ubc10p) as the ubiquitin-conjugating enzyme required for Ubc4p-independent ubiquitination. Whereas Ubc4p-dependent ubiquitination occurs on two lysine residues, Pex4p-dependent ubiquitination neither requires lysine residues nor the NH2-terminal α-NH2 group. Instead, a conserved cysteine residue appears to be essential for both the Pex4p-dependent ubiquitination and the overall function of Pex5p. In addition, we show that this form of ubiquitinated Pex5p is susceptible to the reducing agent β-mercaptoethanol, a compound that is unable to break ubiquitin-NH2 group linkages. Together, our results strongly suggest that Pex4p-dependent ubiquitination of Pex5p occurs on a cysteine residue. Conjugation of ubiquitin to a substrate protein is a well conserved process in eukaryotic cells, sequentially involving an ubiquitin-activating enzyme (E1), 2The abbreviations used are: E1, ubiquitin-activating enzyme; Ubc (E2), ubiquitin-conjugating enzyme; E3, ubiquitin ligase; PTS, peroxisomal targeting signal; MALDI, matrix-assisted laser desorption ionization; K0N, lysine-less NH2-terminal fragment; WT, wild type; IP, immunoprecipitation; Sc, S. cerevisiae. 2The abbreviations used are: E1, ubiquitin-activating enzyme; Ubc (E2), ubiquitin-conjugating enzyme; E3, ubiquitin ligase; PTS, peroxisomal targeting signal; MALDI, matrix-assisted laser desorption ionization; K0N, lysine-less NH2-terminal fragment; WT, wild type; IP, immunoprecipitation; Sc, S. cerevisiae. an ubiquitin conjugating enzyme (E2), and an ubiquitin ligase (E3) (1Hershko A. Ciechanover A. Annu. Rev. Biochem. 1998; 67: 425-479Crossref PubMed Scopus (6736) Google Scholar), whereas ubiquitin chain elongation sometimes requires the action of an additional conjugation factor called E4 (2Koegl M. Hoppe T. Schlenker S. Ulrich H.D. Mayer T.U. Jentsch S. Cell. 1999; 96: 635-644Abstract Full Text Full Text PDF PubMed Scopus (830) Google Scholar). The effect of ubiquitination on a particular protein is, in part, determined by the length of the ubiquitin chain. “Poly-ubiquitination,” i.e. the attachment of 4 or more ubiquitin moieties, typically results in degradation of the substrate by the 26 S proteasome (3Thrower J.S. Hoffman L. Rechsteiner M. Pickart C.M. EMBO J. 2000; 19: 94-102Crossref PubMed Scopus (1291) Google Scholar), whereas “monoubiquitination,” comprising the linkage of 1–3 ubiquitins, usually has a non-proteolytic function, e.g. inducing a change in activity or cellular location (or both) (4Hicke L. Nat. Rev. Mol. Cell. Biol. 2001; 2: 195-201Crossref PubMed Scopus (975) Google Scholar). Many important cellular processes, such as DNA repair, endoplasmic reticulum-retrotranslocation, endocytosis, cell division, and apoptosis are regulated by the poly- or monoubiquitination of participating proteins (for review, see Ref. 5Mukhopadhyay D. Riezman H. Science. 2007; 315: 201-205Crossref PubMed Scopus (934) Google Scholar). Not surprisingly, defective ubiquitination has been implicated in the etiology of important human diseases, such as neurodegenerative disorders and cancer. In the large majority of cases, ubiquitin appears to be conjugated to the ɛ-NH2 group of a lysine residue in the substrate protein, whereas in a limited number of proteins the NH2-terminal α-NH2 group is used as a conjugation site (6Breitschopf K. Bengal E. Ziv T. Admon A. Ciechanover A. EMBO J. 1998; 17: 5964-5973Crossref PubMed Scopus (232) Google Scholar, 7Ben-Saadon R. Fajerman I. Ziv T. Hellman U. Schwartz A.L. Ciechanover A. J. Biol. Chem. 2004; 279: 41414-41421Abstract Full Text Full Text PDF PubMed Scopus (97) Google Scholar, 8Bloom J. Amador V. Bartolini F. DeMartino G. Pagano M. Cell. 2003; 115: 71-82Abstract Full Text Full Text PDF PubMed Scopus (264) Google Scholar). Recently, however, it was reported that the ubiquitination of a lysine-less COOH-terminal tail of the major histocompatibilty complex class I heavy chain was dependent on the presence of a cysteine residue, suggesting that ubiquitin conjugation is not restricted to NH2 groups and that the SH group of a cysteine may also serve as a target (9Cadwell K. Coscoy L. Science. 2005; 309: 127-130Crossref PubMed Scopus (313) Google Scholar). The frequency of this novel mode of ubiquitination and the functional and mechanistic differences (if any) with ubiquitination on NH2 groups remains to be established. A recent addition to the list of processes potentially regulated by ubiquitination is that of the import of proteins into peroxisomes. Peroxisomes are eukaryotic organelles with a wide range of functions, two of which, β-oxidation of long chain fatty acids and H2O2 detoxification, are very well conserved throughout evolution (for review, see Ref. 10van den Bosch H. Schutgens R.B. Wanders R.J. Tager J.M. Annu. Rev. Biochem. 1992; 61: 157-197Crossref PubMed Scopus (732) Google Scholar). Peroxisomes post-translationally import all their matrix enzymes with the aid of a peroxisomal targeting signal (PTS) types one, two, or three. Proteins that contain a PTS signal are recognized in the cytosol by their corresponding cycling receptor (Pex5p for PTS1/3 proteins and Pex7p for PTS2 proteins) and transported to the peroxisomal membrane, where docking takes place. The PTS protein is then released into the peroxisomal matrix and the receptor is recycled to the cytosol for another round of import (reviewed in Ref. 11Purdue P.E. Lazarow P.B. Annu. Rev. Cell Dev. Biol. 2001; 17: 701-752Crossref PubMed Scopus (285) Google Scholar). So far, 32 Pex proteins (called peroxins) have been identified, with around 12 being directly involved in protein import (12Distel B. Erdmann R. Gould S.J. Blobel G. Crane D.I. Cregg J.M. Dodt G. Fujiki Y. J.M. Lazarow P.B. T. A. S. T. D. I. M. J. Cell Biol. PubMed Scopus Google Scholar). of of 12 proteins to a for ubiquitin in the import of the 1992; PubMed Scopus Google Scholar), to the of ubiquitin-conjugating Pex4p is with the peroxisomal D.I. Gould S.J. J. Biol. Chem. Full Text PDF PubMed Google Scholar, A. L. S. J. Cell Biol. 1999; PubMed Google and have that it in the of protein import J.M. Gould S.J. Mol. Cell. Biol. 2000; PubMed Scopus Google Scholar). are and a Gould S.J. J. Cell Biol. 1999; PubMed Scopus Google Scholar, M. M. J. Cell Biol. 2001; PubMed Scopus Google Scholar), the of a class of Cell. 2000; Full Text Full Text PDF PubMed Scopus Google Scholar). two ubiquitinated have been the receptor Erdmann R. Biochem. J. 2004; PubMed Scopus Google Scholar, K. J. Biol. 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The that Ubc4p-dependent ubiquitination a function, Pex5p that is unable to for degradation Erdmann R. Biochem. J. 2004; PubMed Scopus Google Scholar, K. J. Biol. Chem. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar). this Pex5p ubiquitination has been to as in this particular to ubiquitin residues are this of ubiquitination is also in a that Pex4p (Ubc10p) is not In wild cells, on the Pex5p is ubiquitinated the membrane, a process that has been to be of A. T. den M. B. J. Biol. Chem. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar). In of the that two ubiquitin to be not the of this of ubiquitination was called The function of this form of ubiquitination and the enzyme however, A. T. den M. B. J. Biol. Chem. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar). Here we that the two of Pex5p ubiquitination target different residues the NH2-terminal region of the Whereas Ubc4p-dependent ubiquitination occurs on two Ubc4p-independent ubiquitination not lysine residues or the NH2-terminal α-NH2 group. Instead, a conserved cysteine residue in the NH2-terminal is essential for this of this cysteine not Ubc4p-independent also results in a Pex5p. In addition, we show that the enzyme Pex4p is involved in the Ubc4p-independent ubiquitination of Pex5p. and was used for all The used in this and as in Ref. I. Y. B. U. S. A. PubMed Scopus Google on and acids as and on for and then to and and for for and on was to for of and acids as used in this are in the are that are in of are on Pex5p the or and by The ubiquitin was a M. J. Biol. Chem. Full Text PDF PubMed Google used in this den M. G. B. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google M. J. Biol. Chem. Full Text PDF PubMed Google of and in a ubiquitin and wild or of Pex5p with in and in of and and to for was and of and was and a with of and then with of Pex5p and of for 4 with with and and with and and was by the in of and for by and The used for or in our and of the was of the was in of and for of this was used for and of with a COOH-terminal with in β-mercaptoethanol, and was to a of and was by The was and sequentially with and β-mercaptoethanol, and the was and the was an and by and with and to the groups on cysteine residues by and with as in Ref. D. H. 2004; PubMed Scopus Google and by and a with an The used to the of with was as that of conjugated with Pex5p was for with of and reducing agent or of the was of the was in of and of this was of and has been A. T. den M. B. J. Biol. Chem. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar). The NH2-terminal of Pex5p for NH2-terminal of Pex5p the for the docking proteins and is required for the of the protein with the peroxisomal G. A. B. Mol. Biol. Cell. 2000; PubMed Scopus Google Scholar, J. Dodt G. K. A. J. Biol. Chem. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar, A. D. M. Mol. Cell. Biol. 2004; PubMed Scopus Google Scholar). is also essential for the ubiquitination of Pex5p Erdmann R. Biochem. J. 2004; PubMed Scopus Google Scholar, A. T. den M. B. J. Biol. Chem. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar), we a COOH-terminal of of the acids be Ubc4p-dependent wild Pex5p or in a a ubiquitin or a protein and by with In a two be in addition to the major Pex5p and and that are for Ubc4p-dependent ubiquitination in a Erdmann R. Biochem. J. 2004; PubMed Scopus Google Scholar, K. J. Biol. Chem. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar, A. T. den M. B. J. Biol. Chem. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar). of in the cell and that the wild is we have that in wild Pex5p is ubiquitinated in an Ubc4p-independent A. T. den M. B. J. Biol. Chem. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar). this ubiquitinated form of an was ubiquitin A. T. den M. B. J. Biol. Chem. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar). wild or a and ubiquitin to with and by and In the with a for Ubc4p-independent ubiquitination was in wild Pex5p and or not in the Together, results show that the acids of Pex5p contain the target residues of both Ubc4p-dependent and -independent ubiquitination. Ubc4p-dependent of Pex5p and Ubc4p-independent Not on the region involved in the ubiquitination of we to the protein for the not the as it the essential region D. E. S. EMBO J. PubMed Scopus Google Scholar, G. D. B. J. Biol. Chem. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar). the conjugation of ubiquitin to a substrate is usually a lysine residue, we all in the NH2-terminal acids of Pex5p to in of two, or lysine residues a the in their to both of ubiquitination. of and in a in the Pex5p in a or that Ubc4p-dependent ubiquitination was of lysine in a of ubiquitination suggesting that two residues are the of the Pex5p and that lysine is the target for Ubc4p-dependent that lysine to a also as a target is in with recent in the ubiquitination of Pex5p and in a was to be dependent on lysine residues in the NH2-terminal region of the proteins S. L. J. Cregg J.M. S. J. Cell Biol. PubMed Scopus Google Scholar, M. M. 2005; PubMed Scopus Google Scholar). Ubc4p-independent ubiquitination was not in the form of Pex5p or in all the lysine of Pex5p lysine as the of a on a for of the lysine residues in the NH2-terminal region as a target for Ubc4p-independent we a of Pex5p in all the in this to this was a the wild Pex5p and was to the that the Pex5p is functional it be that the in the COOH-terminal region of the Pex5p that was used in may have for ubiquitin we the of ubiquitination of a lysine-less NH2-terminal The was ubiquitinated to a with that of the that the Ubc4p-independent ubiquitination of Pex5p not on lysine A the NH2 for Ubc4p-independent and of of ubiquitin to the α-NH2 group on the NH2 of the has been for a number of a of a linkage A. R. Cell Biol. 2004; Full Text Full Text PDF PubMed Scopus Google Scholar). In Ubc4p-independent ubiquitination of Pex5p also be to the NH2-terminal NH2 group. of Pex5p the the NH2-terminal NH2 group to be a that ubiquitin conjugation on the α-NH2 group. we a COOH-terminal and the the for by and that the majority of to with a of an NH2-terminal with an α-NH2 group was not Instead, a was that to a NH2-terminal with an is the of of the with whereas the in is with an group being to the that this additional is on the residue, as the not the show the that the α-NH2 group on the residue of Pex5p is it for ubiquitin Recently, ubiquitination on a group of a protein was to target the SH group of a cysteine residue (9Cadwell K. Coscoy L. Science. 2005; 309: 127-130Crossref PubMed Scopus (313) Google Scholar). of Pex5p a well conserved cysteine residue in the NH2-terminal region of the protein, whereas the NH2 of all of the also a conserved cysteine the of this cysteine in S. in Ubc4p-independent ubiquitination and Pex5p function, we it with an and or a of the cysteine to the of a on and not a that was by the of the proteins to import proteins of that the of ubiquitination of the was to that of wild Pex5p in a suggest that the not import it with the peroxisomal and is ubiquitinated in an Ubc4p-dependent manner. The of the cysteine into Pex5p and not or all NH2-terminal in a of the ubiquitinated Pex5p with unable to the and that the presence of the cysteine residue is essential for the function of Pex5p and that the of the was not by Ubc4p-dependent ubiquitination of the directly the of the conserved cysteine residue in Ubc4p-independent we different Pex5p and ubiquitination by Whereas the lysine-less Pex5p was this was the conserved cysteine was to The ubiquitinated in the Ubc4p-dependent on Together, the results suggest that the conserved cysteine residue a in the Ubc4p-independent ubiquitination of Pex5p. The NH2-terminal of Pex5p of S. Pex5p and of the the of the NH2-terminal acids in the Pex5p the conserved cysteine residue that this of the protein an important function and may be for Ubc4p-independent ubiquitination of Pex5p. the acids of S. Pex5p with the acids human Pex5p in the conserved cysteine residue is and both the ubiquitination and the of the the Pex5p was modified a with that of and the of all in the Pex5p not ubiquitination and the protein of the on The of the protein in the cell may for this of the cysteine residue in the Pex5p or in the lysine-less in an of ubiquitination and the protein the that the conserved cysteine residue is for both Ubc4p-independent ubiquitination and function of Pex5p. The of Pex5p to results that the conserved cysteine residue in the NH2-terminal of Pex5p function as a site for ubiquitin Conjugation of ubiquitin to a cysteine residue in the of a the group of the residue in ubiquitin and the SH group of the cysteine residue. is also the of linkage and enzymes form with ubiquitin, be by the reducing agent β-mercaptoethanol, whereas a linkage or is not susceptible to (9Cadwell K. Coscoy L. Science. 2005; 309: 127-130Crossref PubMed Scopus (313) Google Scholar). the effect of on the two different of ubiquitinated Pex5p. of of the form of Pex5p with the of ubiquitinated Pex5p In the of ubiquitinated the by this The show that the ubiquitin linkage in the form of Pex5p as a and not as an for the conserved cysteine being the conjugation site for Ubc4p-independent ubiquitination. Ubc4p-independent of Pex5p it has been that ubiquitination of Pex5p in the and is dependent on the enzyme Erdmann R. Biochem. J. 2004; PubMed Scopus Google Scholar, K. J. Biol. Chem. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar, A. T. den M. B. J. Biol. Chem. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar). The for the ubiquitination of Pex5p in wild cells, however, has not been Pex4p (Ubc10p) is a very 1992; PubMed Scopus Google Scholar, D.I. Gould S.J. J. Biol. Chem. Full Text PDF PubMed Google Scholar, Blobel G. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). that Ubc4p-dependent ubiquitination of Pex5p be by of the lysine residues in the NH2-terminal region a of the of Pex4p in Ubc4p-independent ubiquitination. this the ubiquitination of the Pex5p with of wild Pex5p both in a and a The results show that the lysine-less Pex5p is ubiquitinated in the and not in the in wild Pex5p the ubiquitination that Ubc4p-independent ubiquitination of Pex5p requires the presence of and that is not also the ubiquitination of the Pex5p cysteine in the The of the cysteine residue not Ubc4p-dependent ubiquitination the ubiquitinated being a in the with wild Pex5p Ubc4p-dependent ubiquitination to be however, by the of in the NH2-terminal region of this cysteine we that the conserved cysteine residue in Pex5p is essential for Pex4p-dependent in Ubc4p-dependent ubiquitination. The import receptor S. Pex5p is modified by ubiquitin in two of is dependent on the enzyme In this we have that different ubiquitination processes target different residues in the conserved NH2-terminal of Pex5p. Whereas Ubc4p-dependent ubiquitination occurs on two and Ubc4p-independent ubiquitination a well conserved and essential cysteine residue In addition, we show that the ubiquitination requires the enzyme Together, the that two different ubiquitination processes two involved in the of Pex5p. Ubc4p-dependent ubiquitination of S. Pex5p is in and form of ubiquitination has been as and is as a of Pex5p in Erdmann R. Biochem. J. 2004; PubMed Scopus Google Scholar, K. J. Biol. Chem. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar, A. T. den M. B. J. Biol. Chem. 2005; Full Text Full Text PDF PubMed Scopus Google Scholar). The proteins and are to have in the import in the of Pex5p the peroxisomal to the cytosol J.M. Gould S.J. Mol. Cell. Biol. 2000; PubMed Scopus Google Scholar, S. K. Erdmann R. 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J. 2004; PubMed Scopus Google an important for ubiquitination in Pex5p function, as a of and two in and a import The of the two and in Pex5p that are used as of Ubc4p-dependent ubiquitination to the function of this in wild show that of by residues not the function of the protein that Ubc4p-dependent ubiquitination of Pex5p not an essential in the import of is in with in where lysine residues in the NH2-terminal region of H. Pex5p M. M. 2005; PubMed Scopus Google and S. L. J. Cregg J.M. S. J. Cell Biol. PubMed Scopus Google and to be essential for Ubc4p-dependent their effect on protein of the NH2-terminal region of different Pex5p and in all cases, a lysine residue is in the that Ubc4p-dependent ubiquitination of the Pex5p and on is a process that is in in a are the protein is by the proteasome to on the Ubc4p-independent have that Ubc4p-independent ubiquitination of Pex5p also occurs in the NH2-terminal acids of the protein not lysine residues the as to potentially as an attachment site for a well conserved cysteine residue that is essential for both Ubc4p-independent ubiquitination and function of Pex5p. of the cysteine in the lysine-less Ubc4p-independent ubiquitination and in a protein to the by the NH2-terminal region of with that of we that the presence of a cysteine residue, cysteine in human is essential for both the Ubc4p-independent ubiquitination and receptor function and that the ubiquitin linkage in the lysine-less form of Pex5p is susceptible to the reducing agent β-mercaptoethanol, whereas are not The remains that the cysteine not as the conjugation site is involved in the of ubiquitin to a chain of another residue in to the of ubiquitin the site cysteine of an enzyme to the to a lysine of the lysine a whereas of the cysteine in a the residue involved in ubiquitin is the that the ubiquitin to In the of in Pex5p K0N, the α-NH2 group or and residues that form with ubiquitin their The α-NH2 group is a conjugation as we have that it is by is a B. F. J. Mol. Biol. 2003; PubMed Scopus Google Scholar). a of Pex5p remains the of the ubiquitinated Pex5p to that the linkage Pex5p and ubiquitin is not an the of NH2 groups and The to an that be ubiquitin and a or residue as this of linkage also not be susceptible to to the ubiquitinated of Pex5p for to the of the cysteine residue as the conjugation we unable to of modified Pex5p. may be to the of the ubiquitin and Pex5p. strongly suggest that the conserved cysteine residue in Pex5p the conjugation site for So far, ubiquitination on a cysteine has been for the major histocompatibilty complex class I heavy a that is by a ligase (9Cadwell K. Coscoy L. Science. 2005; 309: 127-130Crossref PubMed Scopus (313) Google Scholar). the of cysteine ubiquitination by the cellular ubiquitination a a more lysine residue, is the conjugation site remains to be that the of the ubiquitin Pex5p may an important in the import a process that may a ubiquitin is to a as are more linkages. The of ubiquitinated Pex5p that are in the cell are in with this it be that the of the to may also the in the import is regulated by Ubc4p-independent ubiquitination of Pex5p remains to be two recent suggest a in the of Pex5p the peroxisomal to the and J. M. J. Biol. Chem. 2004; 279: Full Text Full Text PDF PubMed Scopus Google that the NH2-terminal acids of human Pex5p the conserved cysteine are essential for the of the receptor the the group of S. S. J. Biol. Chem. 2007; Full Text Full Text PDF PubMed Scopus Google that the conserved cysteine residue the NH2 of is required for of peroxisomal of the cysteine residue in was not in this it is that this cysteine is also a target for ubiquitination. we far, been unable to show an of the or the the peroxisomal membrane, suggesting that cysteine ubiquitination of Pex5p may be required for another important in the import for The protein import in the and is in with this whereas a in Pex5p be to in a protein import J.M. Gould S.J. Mol. Cell. Biol. 2000; PubMed Scopus Google Scholar). are of function, the conserved of the cysteine residue in both Pex5p and the that cysteine ubiquitination is not restricted to may also in The for Ubc4p-independent that the enzyme Pex4p (Ubc10p) is required for the Ubc4p-independent ubiquitination of Pex5p a the of this ubiquitin-conjugating in the as a ubiquitinated not Erdmann R. Biochem. J. 2004; PubMed Scopus Google Scholar, P.E. Lazarow P.B. J. Biol. Chem. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar). Pex4p to the two of cycling involved in the or PTS2 protein import the that in the of Pex5p and are ubiquitinated by another has on the of Pex4p in ubiquitination of proteins Erdmann R. Biochem. J. 2004; PubMed Scopus Google Scholar, S. L. J. Cregg J.M. S. J. Cell Biol. PubMed Scopus Google Scholar, M. M. 2005; PubMed Scopus Google Scholar). ubiquitination on of the target residues, we show that Pex4p is required for the Ubc4p-independent ubiquitination of Pex5p In a Pex4p-dependent ubiquitination of Pex5p is an that in this by in the and ubiquitinated are in a cell In the of or both Pex4p and whereas in the of ubiquitination The in the ubiquitination with the of Pex5p in wild and wild Pex5p in the that both have a in the i.e. Pex4p-dependent ubiquitination of Pex5p. it was that the attachment of the ubiquitin to a substrate protein occurs on an NH2 group on an lysine or the NH2-terminal residue. the recent that ubiquitin may also be conjugated to a cysteine residue has a of to processes (9Cadwell K. Coscoy L. Science. 2005; 309: 127-130Crossref PubMed Scopus (313) Google Scholar). that ubiquitination on a cysteine is to on proteins involved in that this form of ubiquitination may be more in then recent may be by the of a with an linkage The of more proteins that are ubiquitinated on a cysteine to the of this novel form of ubiquitination. are to for the and and for for and the and of our for with
Williams et al. (Wed,) studied this question.