homologous to the yeast DNA repair gene RAD6 - Europe PMC

The EMBO Journal vol.9 no. 5 pp. 1 431 - 1435, 1990

17 000) is The human ubiquitin carrier protein E2(Mr homologous to the yeast DNA repair gene RAD6 =

Rainer Schneider, Christoph Eckerskorn', Friedrich Lottspeichl and Manfred Schweiger Institut fur Biochemie der naturwissenschaftlichen Fakultdt, Universitiit Innsbruck, Peter Mayr Strasse la, A-6020 Innsbruck, Austria and 'Max-Planck-Institut fur Biochemie, Gentechnologische Arbeitsgruppen, am Klopferspitz 18a, D-8033 Martinsried bei Munchen, FRG

Communicated by M.Schweiger

Components of the ubiquitin conjugating system were purified from human placenta by covalent affinity chromatography on ubiquitin sepharose. In contrast to E2 preparations obtained from rabbit reticulocytes and erythrocytes or Saccharomyces cerevisiae, the placental E2 preparation lacks E2(M, = 14 000) and E2(Mr = 20 000) which are both unique in catalysing the ligase-independent transfer of ubiquitin to histones. A novel technique was employed to detect ubiquitin carrier function of the E2 proteins after SDS-electrophoresis and blotting to nitrocellulose. A cDNA of E2(Mr = 17 000) was isolated from a human cDNA library by screening with a degenerate oligonucleotide whose sequence was based on a partial amino acid sequence obtained from an E2(Mr = 17 000) peptide. Sequence analysis demonstrated an identity of 69% in the primary sequence of human E2(Mr = 17 000) and the protein encoded by the yeast DNA repair gene RAD6, which was recently shown to be an E2 species in yeast. Such a high degree of similarity between the human E2(Mr = 17 000) and the yeast DNA repair enzyme is suggestive of important common structural constraints or roles in addition to ubiquitin carrier activity, since in yeast this function itself is not necessarily dependent on high conservation of primary structure. Key words: DNA repair/homology/primary structure/ renaturation/ubiquitin conjugating system

Introduction Ubiquitin is involved almost 'ubiquitously' in fundamental and diverse cellular processes such as cell cycle control, DNA repair, transcription, protein synthesis, protein degradation, stress response, signal transduction and the immunological response (recently reviewed in Rechsteiner, 1988). It is a protein of 8 500 daltons mol. wt, occurs throughout the entire eukaryotic kingdom and is the most conserved protein known so far. A major function of ubiquitin seems to be the marking of proteins destined for proteolytic elimination (Ciechanover et al., 1984; Hershko et al., 1984). A quite different role for ubiquitin has also been suggested in which it is reversibly joined to another acceptor protein to modulate its function without triggering proteolysis (Finley and Varshavsky, 1985). © Oxford University Press

The enzymatic reactions resulting in the formation of ubiquitin -protein conjugates involve three consecutive steps (Hershko et al., 1983). The first step is the ATP dependent activation of the carboxyl terminus of ubiquitin and is catalysed by the ubiquitin activating (El) enzyme. This activation involves ubiquitin adenylate formation and subsequent covalent binding of the activated polypeptide to a thiol site on the El enzyme. In the second step, ubiquitin is then transferred to specific cysteine residues of the ubiquitin conjugating (or E2) enzymes (also called ubiquitin carrier proteins). In the last step, these E2 proteins may directly transfer ubiquitin by joining the carboxyl terminus of ubiquitin via an isopeptide bond to e-amino groups of lysine residues in acceptor proteins. Alternatively, they may also serve as ubiquitin carriers for another enzyme (E3) called isopeptide ligase, which in turn transfers ubiquitin primarily to proteins that are destined to enter an ubiquitin dependent proteolytic pathway (Hershko et al., 1986). There are other enzymes which are able to de-ubiquitinylate modified proteins (Pickart and Rose, 1985b). To date, a number of in vivo acceptors of ubiquitin have been identified and include histones (Bonner et al., 1988), actin (Ball et al., 1987), cell surface receptors (Siegelman and Weissman, 1988) and the intracellular neurofibrillary tangles in neurodegenerative diseases like Alzheimer's (Perry et al., 1987). The most abundant adduct found in vivo is the monoubiquitin conjugate of the core histone H2A. Although much is known about the structure and function of histone H2A, the role of this modification of an important nuclear protein is still obscure. In rabbit reticulocyte extracts five ubiquitin carrier proteins (E2) with Mr values of 14 000, 17 000, 20 000, 24 000 and 32 000 (Haas and Bright, 1988) have been characterized. Of these the E2(Mr = 14 000) and the E2(Mr = 20 000) are able to donate their ubiquitin moeity to histones H2A and H2B and to cytochrome c in a reaction that does not require isopeptide ligase (E3) (Pickart and Rose, 1985a). Recently, it was shown that the proteins encoded by the yeast DNA repair gene RAD6 and the yeast cell cycle specific gene CDC34 are ubiquitin carrier proteins (Jentsch et al., 1987; Goebl et al., 1988). Mutants of rad6 are extremely vulnerable to DNA damaging agents; they also show defects in mutagenesis and sporulation (Lawrence, 1982; Haynes and Kunz, 1981). Mutants in cdc34 are defective in the transition from GI to the S phase of the cell cycle (Byers and Goetsch, 1973). The involvement of specific ubiquitin conjugating enzymes (E2) in both DNA repair and a regulatory step in the cell cycle in yeast underscores the importance of this modification of nuclear proteins. This prompted us to investigate whether DNA repair in human cells depends on ubiquitinylation and ubiquitin carrier proteins. Here we describe the purification of the E2 enzymes from human placenta, their partial characterization by a newly developed procedure and report the cloning 1431

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and complete sequence of the human E2(Mr = 17 000). The human E2(Mr = 17 000) species is, based on the deduced primary structure, a very close homologue of the product of the yeast DNA repair gene RAD6.

Results Covalent affinity purification of El and E2 components Components of the human ubiquitin conjugating system were purified from human term placenta by covalent affinity chromatography on an ubiquitin sepharose column. Figure 1 shows the result of our standard purification procedure. El is specifically and almost quantitatively released from the column by elution with adenosyl-5'-monophosphate and inorganic pyrophosphate (AMP/PPi) (lane 1). E2 species are released from the ubiquitin column by elution with high concentrations of dithiothreitol (DTT). Major E2 species with Mr of 17 000, 22 000, 23 000 and 27 000 and minor species of Mr 34 000 and 38 000 were found in our preparations from human placenta (Figure 1, lane 3). This pattern of E2 species is quite distinct from that observed with preparations from other tissues. In rabbit reticulocytes the general pattern consists of E2 species with Mr values of 14 000, 17 000, 20 000, 24 000 and 32 000 (Haas and Bright, 1988). Preliminary results suggest that some of the purified E2 enzymes from human placenta exert E3 independent histone ligation activity (not shown) and thus may be counterparts of the reticulocyte E2(Mr = 14 000) and E2(Mr = 20 000) species. No E2 enzymes could be isolated from nuclear placental extracts, even after addition of purified cytosolic El before the ubiquitin -sepharose step.

Ubiquitin acceptor capability of the placental E2 proteins E2 proteins are defined by their ability to accept ubiquitin from an ubiquitin loaded El enzyme. We developed a new rapid detection system for this activity which circumvents tedious purification of E2 proteins. E2 proteins are separated by SDS-PAGE transferred to sheets of nitrocellulose and renatured under special conditions, followed by incubation of the bound proteins with El, ['251]ubiquitin, ATP and Mg2+. Proteins binding ['251]ubiquitin are visualized by autoradiography and their identity as E2 enzymes is established by washing with buffer containing a high concentration of DTT, which results in the selective release of bound ubiquitin from E2 enzymes. In Figure lB (lanes 1-3) it can be seen that all four major E2 species specifically bind ['251]ubiquitin, which in turn can be released by DTT. Under these conditions, the most intense labelling was with the E2(Mr = 17 000) species followed by that of the E2(Mr = 22 000) and E2(Mr = 23 000) species (which are not clearly resolved in this analysis). The E2(Mr = 27 000) is weakly labelled but clearly detectable. No detectable signal was found at positions corresponding to the E2(Mr = 34 000) and E2(Mr = 38 000) species. Isolation and sequencing of an E2(Mr = 17 000) cDNA Screening of a human cDNA library, prepared from HeLa cells (Schneider et al., 1988) with the E2(Mr = 17 000) specific [32P]oligonucleotide probe yielded several cDNA clones; the largest (U18) was chosen for further characteriza-

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Fig. 1. Human placental El and E2 enzyme preparations and activity analysis. (A) SDS-polyacrylamide gels of enzyme preparations from

ubiquitin-sepharose. Gels (20% acrylamide) were stained with Coomassie Blue. Mol. wts correspond to standard proteins run on the same gel. Samples contained 2 fig of AMP/PPj eluate (lane 1), 50 ttg of AMP/PPj eluate (lane 2), or 7.5 14g of DTT eluate (lane 3). (B) Transfers to nitrocellulose sheets of the same preparation as (A, lane 3). Autoradiography of the nitrocellulose sheet after incubation with El, [1251]ubiquitin, ATP, Mg2+ in buffer A and washing with 0.2 M NaCl in buffer A is shown before (lane 1) and after an additional washing step with buffer A containing 0.2 M NaCl and 20 mM DTT (lane 2).

tion by restriction and sequence analysis. The entire nucleotide sequence of U18 (999 bp) is presented in Figure 2. The 5'-untranslated region (38 bp) of UI8 starts with 11 Tresidues, and the following 27 residues are rich in GC (74%). The most probable start codon is at position 39, with an open reading frame extending 456 nucleotides (nt), encoding for a protein containing 152 amino acids. The termination codon is followed by a 502 nt long 3'-untranslated region which contains a rare non-canonical poly(A) addition site (-AATACA-) at position 904-909, 12 bp from the 88 residue long poly(A) tail. The 3'-untranslated region is extremely rich in AT residues (72%, not including the poly(A)). The calculated mol. wt of the predicted protein is 17 312 daltons which is in good agreement with the estimation from SDS-PAGE. The deduced amino acid sequence contains two regions (encoded by nucleotides 201-239 and 459 -485), which are identical to the peptide sequences obtained from purified E2(Mr = 17 000) (one of which was not used to select the cDNA), thus confirming the identity of the clone.

Sequence comparisons Comparison of the deduced primary structure of human E2(Mr = 17 000) with current protein databases yielded significant homologies with two proteins: the gene products of the yeast DNA repair gene RAD6 and the yeast cell cycle gene CDC34 (Figure 3). Both of these proteins are known to possess ubiquitin carrier (E2) function (Jentsch et al., 1987) and show a homology of 38% to each other after introduction of three gaps (Goebl et al., 1988). In contrast, 69% of the amino acids in the human E2(Mr = 17 000) are identical to those in the RAD6 gene product. With inclusion of conservative amino acid replacements, even 84% of the human E2(Mr = 17 000) sequence is conserved within that of the RAD6 gene product, without introduction of any gaps or insertions to optimize the alignment. The similarity of the human E2(Mr = 17 000) sequence to that of the CDC34 product is 52 % after introduction of two gaps. The most conserved region of E2(Mr = 17 000) and the RAD6 gene product is the N-terminal sequence from residues 1 to

Human ubiquitin carrier proteins 38 1 TTTTTTTTTTTCAGACTGACCGCGGGGCAGCTGCGGAC 39 110 ATGTCGACCCCGGCCCGGAGGAGGCTCATGCGGGATTTCAAGCGGTTACAAGAGGACCCACCTGTGGGTGTC M S T P A R R R L M R D F K R L Q E D P P V G V 1 24

111

182

AGTGGCGCACCATCTGAAAACAACATCATGCAGTGGAATGCAGTTATATTTGGACCAGAAGGGACACCTTTT S G A P S E N N I M Q W N A V I F G P E G T P F 25

48

183

254

GAAGATGGTACTTTTAAACTAGTAATAGAATTTTCTGAAGAATATCCAAATAAACCACCAACTGTTAGGTTT E D G T F K L V I E F S E E Y P N K P P T V R F 49

72

255 326 TTATCCAAAATGTTTCATCCAAATGTGTATGCTGATGGTAGCATATGTTTAGATATCCTTCAGAATCGATGG L S K M F H P N V Y A D G S I C L D I L Q N R W 73

96

327

398

AGTCCAACATATGATGTATCTTCTATCTTAACATCAATTCAGTCTCTGCTGGATGAACCGAATCCTAACAGT S P T Y D V S S I L T S I Q S L L D E P N P N S 97

120

399

470

CCAGCCAATAGCCAGGCAGCACAGCTTTATCAGGAAAACAAACGAGAATATGAGAAAAGAGTTTCGGCCATT P A 121

N

S

Q

A

A

Q

L

Y

Q

E

N

K

R

E

Y

E

K

R

V

S

A

I

144

471 542 GTTGAACAAAGCTGGAATGATTCATAATAGACAACTGGTCTGTTAATCTTTTTCATCATTGTTGTGTATAAT V E Q S W N D S End 145 543 614 TTACCTCTCATTAGAAAGGCTAACAAATTTTAAGTGCCACAGGTTTTAAGGATTCTGCAGAAAAAAAAGAAA

615

686

AAAGTCCTTCAGTTTAGAACCTACAAAAGCTTGTGTATCTTGATTAATGTACTTTTTATTGCATGGTGTGAA 687

758

CTAAGTTATTGCTGCATAAATTTGTAATATATCCTGTTTGTATTTTTTTCCAAGTGTATAATGTTGGTGTGG 759

830

AGTTTTCATGACAGAATATACACATTTTGTAAATCTGTACTTTTTTCAAATATTGAATGCCTTATTTTTGAA 831 902 TTCTTTAGATTTTTAAATTGGAGAAAAGCACTTAAAGTTTTTTATATATGAATATTACATGTAAAGCTGTTA 903

AAATACATAACTTCAGTGCA(A)87 Fig. 2. Nucleotide sequence of U18 cDNA and the deduced sequence of the E2(Mr = 17 000) protein. The amino acid sequences identical to those from the tryptic fragments are underlined. The putative polyadenylation site is double underlined and the poly(A) tail of 88 nt is indicated in condensed form.

15, which is identical in these human and yeast proteins. This confirms the choice of the ATG codon at position 39 as the genuine start site, since such a high conservation would be unusual for a 5'-untranslated sequence. Furthermore the amino acid sequence shows very high conservation (21 identities out of 24 residues) in the region of the only cysteine, which is essential for the transthiolation reaction. The major difference between the E2(Mr = 17 000) and the RAD6 protein is found at the carboxy terminus, where the human enzyme lacks 20 amino acids which are mainly acidic and are thought to be involved in substrate recognition (Jentsch et al., 1987; Goebl et al., 1988). At the nucleotide level, a T-rich region 5' to the RAD6 open reading frame (Reynolds et al., 1985) is found at the same position as the tract of 11 Ts 5' to the E2(Mr = 17 000) coding region.

Discussion In this paper we describe the first purification of the ubiquitin conjugating system from human nucleated cells and a partial characterization of the E2 enzymes by a newly developed rapid method. Additionally we report the cloning and complete nucleotide sequence of the cDNA encoding for the E2(Mr = 17 000) component. E2(Mr = 17 000) appears to be the only prominent E2 species common to preparations from mammalian nucleated and enucleated cells. Interestingly, the deduced structure of human E2(Mr = 17 000) reveals a strikingly high homology to the gene product of the yeast DNA repair gene RAD6. This protein was recently shown to be an ubiquitin conjugating enzyme (E2) which has the capacity to ligate ubiquitin to histones

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human E2 (17K) yeast RAD6 yeast CDC34



JWQSTMPKTDL

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50 50 57

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L

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Q

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---------------------

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ILTSIQfNPNSPN L ERdEEqAI dfS YDV ILTSIS X ?N sPANw D K IVE1 VE s P-t K-E %

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95 95 114

152 152 171

Fig. 3. Sequence homologies between human E2(Mr = 17 000) and yeast RAD6 and CDC34 gene products. The amino acid sequences are numbered on the right side. Dashes indicate gaps introduced into the sequences of the E2(Mr = 17 000) protein and the RAD6 gene product to optimize their alignment with the CDC34 product. Identical amino acids occurring in two or three sequences at the same alignment position are boxed. The C-terminal extensions of the yeast proteins, which have no counterpart in the human E2(Mr = 17 000) protein are not shown.

in an E3 independent manner (Jentsch et al., 1987). The high degree of homology between the human E2(Mr = 17 000) protein and yeast RAD6 gene product is probably not a formal requirement to preserve the mere enzymatic activity or substrate specificity. This is supported by the observation that the two known yeast ubiquitin conjugating enzymes (E2), the RAD6 and CDC34 gene products possess the same enzymatic activity and a similar substrate specificity, yet they show only a moderate homology. Thus, considering the vast evolutionary distance between man and yeast, such a high conservation (even higher than for the corresponding cytochrome c's) suggests strong structural constraints essential to an important common function. The gene product of RAD6 plays a central role in one of the epistasis groups of mutations affecting DNA repair (Lawrence, 1982). The rad6 mutants show a markedly pleiotropic phenotype including retarded growth, impaired induced mutagenesis, defective sporulation and extreme vulnerability to X-rays, UV-light, chemicals noxious to DNA and trimethoprim (Lawrence, 1982; Tuite and Cox, 1981; Haynes and Kunz, 1981). Thus involvement in DNA repair seems to be a likely candidate for such a common function of the yeast and human ubiquitin conjugating enzymes (E2). This raises the possibility that the E2(Mr = 17 000) or other human E2 species are involved in one of the hereditary human DNA repair diseases.

Our systematic approach to the study of the molecular biology and biochemistry of the human ubiquitin conjugating system has not only yielded structural data that provide important clues to the function of this system but has also provided us with the molecular reagents to probe for alterations of the ubiquitin conjugating system in human diseases.

Materials and methods Preparation of placental extract Two human placentas obtained within 60 min of normal delivery were stripped of the membranes, cut into pieces of -5- 10 g, and washed thoroughly with cold 0.15 M KCI containing 12 mM mercaptoethanol. The washed placenta pieces (800 g) were mixed with 1.6 1 of buffer H (20 mM Tris, pH 7.2, 5% glycerol, 10 mM DTT, 20 mM mercaptoethanol, 1 mM EDTA) and homogenized at 4°C in a Waring blender three times at full speed for 90 s. Preparation of the fraction Il-equivalent The homogenate was centrifuged (10 000 g, 10 min) and the supernatant was filtered through cheesecloth. The cleared lysate was filtered through 1434

200 g (dry wt) of diethylaminoethyl (DEAE) 52 cellulose (Whatman, Maidstone, UK), previously equilibrated with buffer H and settled in a Buchner funnel. The DEAE cellulose was washed with 10 vols of buffer H and the fraction H-equivalent (Hershko et al., 1983) was eluted with 2 vols of buffer H containing 0.5 M KCI and no thiol compounds. Protein containing fractions were pooled (300 ml) and dialysed against buffer A (50 mM Tris, pH 7.5, 0.2 mM DTT).

Affinity purification The affinity purification of the ubiquitin conjugating system was essentially the same as published by Hershko et al. (1983) and Ciechanover et al. (1982). Starting from two placentas, 3.2 mg of El enzyme and 650 ytg protein in the E2 fraction were obtained. The El preparation is contaminated with a small amount of 'trapped' E2 molecules that, in a reversal of the normal reaction, transfer ubiquitin to E I and are thereby released from the column (Haas and Bright, 1988). As can be seen in Figure 1, lane 2, the major trapped E2 is the E2(Mr = 23 000) species. Electrophoresis and renaturation of the E2 enzymes after blotting of nitrocellulose SDS-PAGE was essentially according to Laemmli (1970) using 20% gels. Blotting of the E2 enzymes run on 20% gels was done in a semidry chamber (Semi-phor TE 70, Hoefer Scientific Instruments, San Francisco, CA) using the standard SDS -electrophoresis buffer containing 1 mM mercaptoethanol and nitrocellulose BA85 (Schleicher und Schull, Dassel, FRG). Transfer time was 1.5 h using 2.5 mA/cm2 at 4°C. After the blotting procedure the nitrocellulose sheet was put in buffer A containing 2% commercial skim milk powder and shaken for 2 h at 4°C. The ubiquitin transthiolation reaction was carried out in buffer A containing 2 mM ATP, 5 mM MgCl2, 4 iLg/ml El and 3 /ig [1251]ubiquitin (106 c.p.m./Ag)/ml for 3 h at room temperature. Afterwards, the nitrocellulose sheet was washed thoroughly with buffer A containing 200 mM NaCl. Thiol-bound ubiquitin was removed by increasing the DTT concentration to 20 mM. The nitrocellulose sheets were exposed to X-ray film (Cronex IV, DuPont, Bad Homburg, FRG) with an intensifying screen at -80°C. Amino acid sequence analysis and oligonucleotide probe design E2(Mr = 17 000) was cleaved by trypsin directly in the polyacrylamide gel and internal peptides were obtained by elution and reverse HPLC as described (Eckerskom and Lottspeich, 1989). Amino acid sequence analysis of the peptides was performed using a gas phase sequencer 470A (Applied Biosystems, Foster City, CA) equipped with a prototype isocratic HPLC system for the identification of the phenylthiohydantoin amino acids (Lottspeich, 1985; Eckerskom et al., 1985). Two sequences were obtained: LVIEFSEEYPNKP and VSAIVEQSW. The underlined sequence was used to design the degenerate oligonucleotide probe mixture:

GA(G,A)GA(G,A)TA(T,C)CCNAA(T,C)AA(G,A)CC. cDNA cloning and DNA sequencing Cloning and screening procedures were according to standard protocols (Maniatis et al., 1982). cDNA inserts were subcloned into the Bluescript plasmid (Stratagene, San Diego, CA) and sequences were determined by the dideoxy chain termination method (Sanger et al., 1977).

Human ubiquitin carrier proteins

Sequence comparison The MIPs protein database of the Max-Planck-Institut in Martinsried, Munich was searched for related protein sequences and these were compared to that of the human E2(Mr = 17 000) using the FAST-P program (Lipman and Pearson, 1985).

Acknowledgements We are very grateful to Dr Robert Shoeman from the Max-Planck-Institut, Ladenburg for carefully and critically reading and improving this manuscript.

References Ball,E., Karlic,Ch.C., Beall,C.J., Saville,D.L., Sparrow,J.C., Bullard,B. and Fyrberg,E.A. (1987) Cell, 51, 221-228. Bonner,M.W., Hatch,C.L. and Wu,R.S. (1988) In Rechsteiner,M. (ed.), Ubiquitin. Plenum, New York, pp. 157-172. Byers,B. and Goetsch,L. (1973) Cold Spring Harbor Svmp. Quant. Biol., 38, 123-131. Ciechanover,A., Elias,S., Heller,H. and Hershko,A. (1982) J. Biol. Chein., 257, 2537-2542. Ciechanover,A., Finley,D. and Varshavsky,A. (1984) Cell, 37, 57-66. Eckerskom,Ch. and Lottspeich,F. (1989) Chromatographia, 28, 92-94. Eckerskorn,Ch., Mewes,W., Goretzki,H., Lottspeich,F. (1988) Eur. J. Biochem., 176, 509-519. Finley,D. and Varshavsky,A. (1985) Trends Biochem. Sci., 10, 343-346. Goebl,M.G., Yochem,J., Jentsch,S., McGrath,J.P., Varshavsky,A. and Byers,B. (1988) Science, 241, 1331-1335. Haas,A.L. and Bright,P.M. (1988) J. Biol. Chem., 263, 13258-13267. Haynes,R.H. and Kunz,B.A. (1981) In Strathern,J., Jones,E. and Broach,J. (eds), The Molecular Biology of the Yeast Saccharomvces cerestisiae: Life Cycle and Inheritance. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY, pp. 371-414. Hershko,A., Heller,H., Elias,S. and Ciechanover,A. (1983)J. Biol. Chem., 258, 8206-8214. Hershko,A., Leshinsky,E., Ganoth,D. and Heller,H. (1984) Proc. Natl. Acad. Sci. USA, 81, 1619-1623. Hershko,A., Heller,H., Eytan,E. and Reiss,Y. (1986)J. Biol. Chem., 261, 11992-11999. Jentsch,S., McGrath,J.P. and Varshavsky,A. (1987) Nature, 329, 131 -134. Laemmli,U.K. (1970) Nature, 227, 680-685. Lawrence,C.W. (1982) Adv. Genet., 21, 173-254. Lipman,D. and Pearson,W. (1985) Science, 227, 1435-1441. Lottspeich,F. (1985) J. Chromatogr., 326, 321-327. Maniatis,T., Fritch,E.F. and Sambrook,J. (1982) Molecular Cloning. A Laboratorn Manual. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY. Perry,G., Friedman,R., Shaw,G. and Chau,V. (1987) Proc. Natl. Acad. Sci. USA, 84, 3033-3036. Pickart,C.M. and Rose,I.A. (1985a) J. Biol. Chem., 260, 1573-1581. Pickart,C.M. and Rose,I.A. (1985b) J. Biol. Chem., 260, 7903-7910. Rechsteiner,M. (ed.) (1988) Ubiquitin, Plenum, New York. Reynolds,P., Weber,S. and Prakash,L. (1985) Proc. Natl. Acad. Sci. USA, 82, 168-172. Sanger,F., Nicklen,S. and Coulson,A.R. (1977) Proc. NatI. Acad. Sci. USA, 74, 5463-5467. Schneider,R., Schneider-Scherzer,E., Thurnher,M., Auer,B. and Schweiger,M. (1988) EMBO J., 7, 4151-4156. Siegelman,M. and Weissman,I.L. (1988) In Rechsteiner,M. (ed.), Ubiquitin, Plenum, New York, pp. 239-269. Tuite,M.F. and Cox,B.S. (1981) Mol. Cell. Biol., 1, 153-157.

Receiv'ed on October 24, 1989; revised on Februarn 8, 1990

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