The BAL-binding Protein BBAP and Related Deltex Family Members Exhibit Ubiquitin-Protein Isopeptide Ligase Activity*

Members of the DTX (Deltex) family act as Notch sig- naling modifiers and may also regulate transcription through interactions with specific transcription factors. DTX proteins have a basic N terminus; a central proline-rich region; and a C-terminal RING finger domain, a motif often found in ubiquitin-protein isopeptide ligases (E3). Recently, we identified and characterized a unique diffuse large B-cell lymphoma risk-related gene named BAL (B aggressive lymphoma). Using a yeast two-hybrid screen for BAL-binding partners, we have now identified a novel protein termed BBAP (B-lymphoma- and BAL-associated protein). Although BBAP has a unique N terminus, the C-terminal region is highly homologous to that of DTX family members. Herein, we report that BBAP and the human family of DTX proteins (DTX1, DTX2, and DTX3) function as E3 ligases based on their capacity for self-ubiquitination. DTX family members homodimerize and heterodimerize in vivo , suggesting that physical interactions between various DTX family members modify E3 activity and/or substrate availabil-ity. Consistent with this idea, BBAP and DTX1 associate via their unique N termini, resulting in enhanced self-ubiquitination. Diffuse

Diffuse large B-cell lymphoma (DLBCL) 1 is the most common lymphoid malignancy in adults and a clinically heterogeneous disorder (1). As part of an effort to identify genes associated with the unique behavior of curable versus fatal DLBCLs, we recently isolated and characterized a gene termed BAL (B aggressive lymphoma) that is more highly expressed in fatal tumors (2). BAL is a nuclear protein with a unique C terminus and a duplicated N-terminal domain homologous to the non-histone region of histone macro-H2A (2), a polypeptide that associates with the inactive X chromosome (3) and that may play a role in gene silencing (4).
We employed yeast two-hybrid screening to identify a BALbinding protein termed BBAP (B-lymphoma-and BAL-associated protein), described herein. Although BBAP contains a unique N terminus, the C terminus is highly homologous to the conserved C termini of DTX (Deltex) family members (5)(6)(7), all of which include a classic RING finger domain.
DTX polypeptides participate in the Notch signaling pathway, which controls cell fate determination in multicellular animals (8,9). In the fly, DTX augments Notch signals that are dependent on the downstream transcription factor Su(H) (7) and also participates in less well understood Su(H)independent Notch signaling (10). Four murine DTX proteins, DTX1, DTX2, DTX2⌬E (a splice variant), and DTX3, have been described previously (6). In mammalian systems, ectopic expression of DTX perturbs myogenesis and neurogenesis (6) and lymphogenesis (11) (cell fates also influenced by Notch) and alters the transcriptional activity of E2A (6,7), the general coactivator p300 (12), and the Su(H) homolog CSL (7). Of particular interest, DTX1 appears to promote B-cell development at the expense of T-cell development (11), and its close homolog, DTX2, has been identified recently as a site of recurrent retroviral integration in murine B-cell lymphomas (13). How DTX influences B-cell development and transformation, as well as other diverse phenotypes, is not understood.
Drosophila DTX and human DTX1 include a basic N terminus, which binds the ankyrin repeats of intracellular Notch; a central proline-rich region; and a C-terminal RING finger domain (5,7). The RING finger consensus sequence has eight conserved cysteines and histidines that coordinate two zinc ions in a "cross-braced" fashion (14). Recently, certain RING finger proteins were found to be ubiquitin-protein isopeptide ligases (E3) that mediate the transfer of ubiquitin to themselves and specific heterologous substrates (15)(16)(17). The multistep process of ubiquitination is hierarchical, beginning with an initial activation and transfer of ubiquitin from a common ubiquitin-activating enzyme (E1) to ubiquitin-conjugating enzymes (E2). Thereafter, a given E2 interacts with a specific substrate and E3, which catalyzes the transfer of ubiquitin to the substrate (18). Although polyubiquitinated proteins are often targeted for proteasomal degradation, ubiquitination regulates additional cellular functions that are not directly related to proteasomal degradation (19).
Given the possible roles of DTX family members in B-lineage commitment and transformation, we systematically evaluated BBAP and human DTX family members for E3 activity and heterodimerization. We found that all members of this group are E3 ligases and that heterodimerization of BBAP and DTX1 enhances this activity. * This work was supported by a Lymphoma Research Foundation fellowship (to K. T.), the Leukemia and Lymphoma Society of America (6624-01 to M. A. S. and 3093-00 to R. C. T. A.), and the V Foundation (to R. C. T. A.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. The

EXPERIMENTAL PROCEDURES
Yeast Two-hybrid Screening-The C-terminal half (aa 486 -854) (2) of BAL was used as bait in a yeast two-hybrid screen. Plasmids encoding candidate BAL-binding proteins were rescued and confirmed by retransformation. Inserts from positive clones were sequenced, leading to identification of a partial cDNA encoding a novel BAL-binding protein, BBAP. A full-length BBAP cDNA was identified in an activated B-cell plasmid library (20) with a 5Ј BBAP 400-bp biotinylated probe using the ClonCapture library selection kit (Clontech).
Cloning of Human DTX2A, DTX2B, and DTX3-Human DTX2A, DTX2B, and DTX3 were cloned by reverse transcription-PCR. Human DTX2 PCR primers were designed from the sequences of mouse dtx2 (NCBI accession number AB015423), mouse dtx2⌬E (NCBI accession number AB015424), and human KIAA1528 (NCBI accession number AB040961) and human genome sequence (NCBI accession number NT_007834). Human DTX3 PCR primers were designed according to the mouse dtx3 sequence (NCBI accession number AB015425) and human genome sequence (NCBI accession number NT_009410). All human dTX PCR products (DTX2A, DTX2B, and DTX3) were subcloned into the pCR4 Blunt-TOPO vector (Invitrogen) and verified by sequencing.
Immunoprecipitation and Western Blotting-After Ϸ48 h of transfection, 293 cells were lysed in buffer containing 150 mM NaCl, 50 mM Tris (pH 7.4), 1% Nonidet P-40, 1 mM sodium orthovanadate, and protease inhibitor mixture (Complete, Roche Applied Science). Following centrifugation and preclearing of cell lysates with protein G-Sepharose (Amersham Biosciences), the supernatants were subsequently incubated with the appropriate antibodies and protein G-Sepharose. Immune complexes were washed, resuspended in SDS-PAGE sample buffer, heated at 95°C, size-fractionated by SDS-PAGE, and transferred to polyvinylidene difluoride membranes (Millipore Corp., Bedford, MA). Membranes were immunostained with the appropriate primary and secondary antibodies and developed using a chemiluminescent method (ECL, Amersham Biosciences).
Antibodies-The murine anti-BBAP monoclonal antibody (clone 28F-3E1) was generated following repetitive immunization with purified BBAP-GST fusion protein according to standard protocols. The previously described rabbit anti-BAL polyclonal antibody (2) was affinitypurified according to standard protocols. The following antibodies were purchased from the indicated commercial sources: mouse anti-FLAG monoclonal antibody M2 (Sigma), mouse anti-Myc monoclonal antibody 9E10 and mouse anti-GST monoclonal antibody B-14 (Santa Cruz Biotechnology, Santa Cruz, CA), mouse anti-HA monoclonal antibody HA.11 (Covance Research Products, Richmond, CA), and horseradish peroxidase-conjugated sheep anti-mouse IgG antibody and horseradish peroxidase-conjugated donkey anti-rabbit IgG antibody (Amersham Biosciences). In Vivo Ubiquitination Assay-HA-tagged ubiquitin plasmids were cotransfected with FLAG-tagged BBAP or DTX plasmid into 293 cells. Forty hours later, cells were treated with the proteasome inhibitor MG-132 (Calbiochem) at a final concentration of 25 M for an additional 6 h. FLAG-tagged proteins were immunoprecipitated, size-fractionated by SDS-PAGE, and immunostained using anti-HA or anti-FLAG antibody.
Recombinant Protein Preparation-FLAG-tagged wild-type BBAP and Myc-tagged DTX1, DTX2A, DTX2B, and DTX3 cDNAs were subcloned into pGEX-4T-2 vectors (Amersham Biosciences). A FLAGtagged RING finger deletion mutant of BBAP, BBAP RFdel , was generated by two-step PCR. In the first step, full-length BBAP cDNA was used as a template to generate two separate products, BBAP-(1-560) and BBAP-(600 -740), using the appropriate primers (the BBAP Nterminal primer and 5Ј-ctggatgcccttttccttcttgtccagttctgaggc-3Ј or the BBAP C-terminal primer with FLAG sequence and 5Ј-gacaagaaggaaaagggcatccagacttcctatggtattcagaaagg-3Ј) with overlapping ends (underlined). Two PCR products were purified, mixed, and used as templates in the second step, in which BBAP N-and C-terminal FLAG primers were used to amplify BBAP RFdel . The final product was subcloned into the pGEX-4T-2 vector. GST fusion proteins were expressed in Escherichia coli BL21 cells (Amersham Biosciences) and purified from bacterial cell lysates using standard protocols.

RESULTS
Identification of the BAL-binding Protein BBAP-In a yeast two-hybrid screen using a C-terminal portion of BAL as bait, we identified a novel interacting protein termed BBAP (Blymphoma-and BAL-associated protein). The full-length BBAP cDNA encodes a 740-aa protein with a unique 560-aa N terminus containing two potential nuclear localization signals (aa 20 -26 and 462-478) (23) and a possible nuclear export signal (aa 325-334) (24) (Fig. 1A). The BBAP C terminus is highly homologous to the conserved C termini of human DTX1 and the related murine proteins DTX1, DTX2, and DTX3, including a classic RING finger domain (BBAP aa 561-599) (Fig. 1A). BBAP is expressed as a single major transcript of 6.5 kb (data not shown).  (7) and murine DTX family members (6). The N-terminal domains 1 of DTX1, DTX2A, DTX2B, and Drosophila DTX (turquoise) bind to Notch ankyrin repeats. Human DTX3 has a short unique N-terminal domain (orange), and BBAP contains a longer unique N-terminal region (yellow). Domains 2 of DTX1, DTX2A, DTX2B, DTX3, and Drosophila DTX contain a proline-rich region (green) that is not found in BBAP. DTX2B lacks the fourth of nine predicted DTX2 exons (dark green region in DTX2A). The C-terminal domain 3 (purple) is highly conserved in all DTX proteins and BBAP; each C-terminal region includes a classic RING finger domain (RING-H2: DTX1, DTX2A, DTX2B, and Drosophila DTX (light pink); or RING-HC: DTX3 and BBAP (dark pink)). In Drosophila DTX, domains 1-3 are separated by opa repeats (gray) (5). B, C-terminal amino acid sequence alignment of DTX1, DTX2A, DTX3, BBAP, and Drosophila DTX. Alignments were generated using the ClustalW Version 1.8 multiple sequence alignment program (33) and refined with BOXSHADE Version 3.21 (available at www.ch.embnet.org/software/BOX_form.html). Identical residues are shown in red, and similar residues are shown in blue. The conserved cysteine and histidine residues of the RING finger motifs are indicated with pink boxes. The asterisk denotes the fifth residues of the DTX1, DTX2A, and Drosophila DTX RING-H2 fingers (histidines) and the DTX3 and BBAP RING-HC fingers (cysteines). Residues that are shared by BBAP and DTX3 (but not by DTX1 and DTX2A) are highlighted in yellow.

Confirmation of BAL/BBAP Binding and Identification of the Interaction Domains-To confirm the association between BAL
and BBAP, we immunoprecipitated BAL and associated proteins from a panel of DLBCL cell lines. Thereafter, these proteins were size-fractionated and immunoblotted with a newly developed anti-BBAP monoclonal antibody (Fig. 1B, upper  panel) or anti-BAL antiserum (lower panel). The Ϸ89-kDa BBAP protein was readily detected in BAL immunoprecipitates (Fig. 1B, upper panel) in quantities comparable to those of the BAL protein, which has a similar size (lower panel).
To identify the domains responsible for BAL/BBAP binding, FLAG-tagged BAL and HA-tagged BBAP constructs were cotransfected into 293 cells and subsequently immunoprecipitated and immunoblotted with the relevant antibodies. In separate assays, FLAG-tagged full-length, N-terminal, and C-terminal BAL proteins were assessed for association with HA-tagged full-length, N-terminal, and C-terminal BBAP proteins (Fig. 1C). When either FLAG-tagged full-length or Cterminal BAL was immunoprecipitated, full-length or N-terminal BBAP was co-associated with the BAL protein (Fig. 1C,  upper left panel, lanes 1, 2, and 5). Similarly, when either HA-tagged full-length or N-terminal BBAP was immunoprecipitated, full-length or C-terminal BAL was co-associated with BBAP (Fig. 1C, lower left panel, lanes 1, 2, and 5). In contrast, neither the N-terminal BAL nor the C-terminal BBAP polypeptide coprecipitated with their respective partner proteins (Fig.  1C, upper and lower left panels, lanes 3 and 4), although these proteins were readily detectable in transfected cells (upper right panel, lane 8; and lower right panel, lane 9). Taken together, these data confirm that BAL and BBAP co-associate in vivo and identify the BBAP N terminus (Fig. 1A) and the BAL C terminus (2) as the interaction domains.
DTX Family Members and BBAP-Although the N-terminal BBAP interaction domain is unique, the C terminus of BBAP is highly homologous to those of human DTX1 (7) and the recently described murine family members DTX1, DTX2, DTX2⌬E, and DTX3 (6). To analyze the function of the conserved C-terminal BBAP/DTX domain, we first cloned and characterized the human homologs of murine DTX2, DTX2⌬E, and DTX3, hereafter termed DTX2A, DTX2B, and DTX3, respectively (Fig. 2).
Human DTX family members have three distinct domains based on the analogous functional motifs in Drosophila DTX (5, 7) (Fig. 2A). The N-terminal domain 1 can associate with Notch ankyrin repeats, whereas domain 2 contains a proline-rich region with potential SH3 (Src homology-3) domain-binding sites. The C-terminal domain 3 includes a classic RING finger domain and the region of BBAP homology ( Fig. 2A).
Human DTX1, DTX2A, and DTX2B are similarly organized, although DTX2B lacks the fourth of nine predicted DTX2 exons (NCBI accession number NT_007833.10) within the proline-rich domain 2. The N-terminal regions of human DTX1, DTX2A, and DTX2B and Drosophila DTX all contain two WWE motifs, a postulated protein/protein interaction domain (25). DTX3 has a truncated domain 1 that lacks homology to other DTX family members, but retains a proline-rich region ( Fig. 2A). As noted, the unique BBAP N terminus does not contain the WWE and proline-rich domains of DTX family members (Fig. 2A).
The amino acid sequences of the highly conserved C termini of BBAP, human DTX family members, and Drosophila DTX are aligned in Fig. 2B. Each of these proteins includes a classic RING finger domain of either the RING-H2 or RING-HC subtype (26). DTX1, DTX2, and Drosophila DTX have RING-H2 fingers with histidines in the fourth and fifth positions; in contrast, DTX3 and BBAP contain RING-HC fingers with a single histidine in the fourth position (Fig. 2B). Of interest, the additional C-terminal sequences of the RING-HC-containing DTX3 and BBAP proteins are also more similar to each other than to those of the other RING-H2-containing DTX family members (DTX1, DTX2, and Drosophila DTX) (Fig. 2B).
E3 Activity of BBAP-Certain RING finger proteins function as E3 ligases, mediating the transfer of ubiquitin to heterologous substrates and to the RING finger proteins themselves (17). To determine whether BBAP functions as an E3, we performed ubiquitination assays in cultured cells. In brief, FLAG-tagged BBAP and HA-tagged ubiquitin were cotransfected into 293 cells, and proteasome activity was inhibited with MG-132. Cell lysates were immunoprecipitated with anti-FLAG (BBAP) antibody, size-fractionated, and immunoblotted with anti-HA (ubiquitin) antibody (Fig. 3A, left panel) or anti-FLAG antibody (right panel). The cotransfection of BBAP and ubiquitin resulted in a dramatic increase in high molecular mass polyubiquitinated proteins (Fig. 3A, left panel), suggesting that BBAP increased ubiquitination and was a possible target for self-ubiquitination.
To directly assess the E3 activity of BBAP, we performed in vitro assays in which self-ubiquitination of recombinant BBAP protein was determined. In the presence of E1, E2, and ubiquitin, recombinant BBAP was heavily polyubiquitinated, confirming that it functions as an E3 capable of self-ubiquitination in vitro (Fig. 3B).
To determine the role of the RING finger domain in BBAP E3 activity, we performed additional in vitro ubiquitination assays using either the wild-type recombinant BBAP protein or a RING finger deletion mutant (BBAP RFdel ) and the required additional components (E1, E2, and biotinylated ubiquitin) (Fig. 3C). Whereas wild-type BBAP exhibited obvious E3 activity, BBAP RFdel was inactive (Fig. 3C), confirming that the RING finger domain is required for BBAP E3 function.
Each E3 recognizes specific substrates in cooperation with a common E1 and a smaller subset of E2 enzymes (18). To identify the preferred E2 for BBAP, we first analyzed the coprecipitation of BBAP and individual E2 enzymes (Ubc4, UbcH5a, UbcH5b, UbcH5c, UbcH6, and UbcH7) in transiently transfected 293 cells (Fig. 4A, left panels). Although each of these E2 proteins was expressed at readily detectable levels in whole cell lysates from transfected cells (Fig. 4A, upper right panels), BBAP co-immunoprecipitated only with UbcH5 family members (left panels).
Thereafter, we evaluated the E2 activity of UbcH5 family members for BBAP using recombinant enzymes in an in vitro ubiquitination assay (Fig. 4B). Although each UbcH5 enzyme was active in this assay, the most striking high molecular mass polyubiquitination occurred when the E2 enzyme UbcH5a was used with BBAP (Fig. 4B).
E3 Activities of DTX1, DTX2A, DTX2B, and DTX3-Given the high degree of C-terminal homology between BBAP and human DTX family members, we next investigated whether DTX1, DTX2, and DTX3 also function as E3 ligases using tagged recombinant DTX-GST fusion proteins in the in vitro ubiquitination assay (Fig. 5). As indicated, DTX1, DTX2A, DTX2B, and DTX3 all exhibited E3 activity as assessed by self-ubiquitination (Fig. 5, A-C). Like BBAP, the DTX1, DTX2, and DTX3 E3 ligases also functioned most efficiently in conjunction with the UbcH5a E2 enzyme (data not shown).
BBAP Homodimerizes and Interacts with DTX through Its Unique N Terminus-The BBAP RING finger E3 protein was initially identified on the basis of its N-terminal interaction with BAL (Fig. 1C). Additional RING finger proteins, including DTX family members (6,12,27), have been reported to dimerize in ways that alter their biological activity. For these reasons, we investigated whether BBAP might homodimerize or heterodimerize with other DTX family members.
In initial experiments, FLAG-tagged wild-type BBAP and HA-tagged wild-type, N-terminal, and C-terminal BBAP polypeptides were coexpressed in 293 cells (Fig. 6A, upper and  lower panels). FLAG-tagged wild-type BBAP coprecipitated with HA-tagged wild-type and N-terminal BBAP proteins ( 6A, upper left panel), and HA-tagged wild-type and N-terminal BBAP proteins coprecipitated with FLAG-tagged wild-type BBAP (lower left panel). In contrast, C-terminal BBAP did not associate with wild-type BBAP (Fig. 6A, upper and lower left panels), indicating that BBAP homodimerization requires the unique N terminus.
To determine whether BBAP also interacts with DTX family members, we coexpressed FLAG-tagged wild-type, N-terminal, and C-terminal BBAP polypeptides and Myc-tagged DTX1 in 293 cells (Fig. 6B, upper and lower panels). FLAG-tagged wildtype and N-terminal BBAP proteins coprecipitated with Myctagged wild-type DTX1 (Fig. 6B, upper left panel); similarly, Myc-tagged DTX1 coprecipitated with wild-type and N-terminal (but not C-terminal) BBAP proteins (lower left panel). Taken together, these data demonstrate that BBAP/DTX1 heterodimerization also requires the unique BBAP N terminus.
DTX1 Enhances BBAP Ubiquitination-BBAP functions as an E3 via its DTX-homologous C-terminal RING finger domain and interacts with DTX1 via its unique N terminus, suggesting that BBAP/DTX1 binding may have functional consequences. To address this possibility, we compared the self-ubiquitination activity of BBAP in the presence and absence of DTX1 (Fig. 7). Myc-tagged DTX1-GST protein was added to FLAG-tagged wild-type BBAP-GST or BBAP RFdel -GST protein (Fig. 7); wildtype BBAP protein was also incubated separately. Thereafter, the additional required components (E1, E2, and biotinylated ubiquitin) were added, and samples were size-fractionated and serially immunoblotted with anti-FLAG (BBAP) antibody and peroxidase-conjugated avidin (ubiquitin) (Fig. 7). In this assay, wild-type BBAP underwent the expected polyubiquitination, whereas BBAP RFdel did not (Fig. 7, left panel, lanes 1 and 4). In comparison with the wild-type BBAP homodimer, the wild-type BBAP/DTX1 heterodimer exhibited increased BBAP polyubiquitination (Fig. 7, left and right panels, lanes 1 versus lanes 3). Taken together, these data indicate that the BBAP/DTX1 heterodimer is a better E3 and substrate for self-ubiquitination compared with the BBAP homodimer. DISCUSSION We have cloned and characterized the BAL-binding protein BBAP, which has highly significant C-terminal homology to DTX family members. However, in contrast to DTX family members, BBAP includes a unique N terminus and lacks a central proline-rich region. The BBAP and DTX C termini contain classic RING-HC or RING-H2 finger domains. Using in vivo and in vitro ubiquitination assays, we have shown that BBAP functions as an E3 and promotes its self-ubiquitination in a RING finger-dependent manner. In analogous ubiquitination assays, we demonstrated that DTX1, DTX2, and DTX3 also function as E3 ligases. In additional binding studies, DTX1 and BBAP formed heterodimers via their respective N termini in a manner that enhanced BBAP-mediated self-ubiquitination.
In lymphoid progenitors, DTX acts as a negative modulator of Notch signaling (11). However, the observed differences in DTX activity in certain in vitro and in vivo assays suggest that DTX function may be dose-and cell type-dependent and tightly regulated. For these reasons, the identification of DTX family members as E3 ligases and targets for self-ubiquitination is of particular interest. Recent studies highlight the pivotal role of ubiquitination in regulating Notch-mediated signaling (28). Four known E3 ligases regulate Notch itself, Notch ligands, or known Notch antagonists (28). That DTX family members also function as E3 ligases further emphasizes the role of ubiquitination in regulating Notch signaling.
The striking C-terminal homology between BBAP and human DTX family members prompted our analysis of their common functions. However, the BBAP C terminus is more closely related to the DTX3 C terminus than to the DTX1 and DTX2 C termini. Of interest, neither BBAP nor DTX3 contains the DTX1/2 N terminus, which physically interacts with Notch ankyrin repeats, suggesting that BBAP and DTX3 have unique functions that are not shared by DTX1 and DTX2. Given the demonstrated role of the BBAP N terminus as an interaction domain (Figs. 1C and 6), BBAP may have additional unique binding partners. FIG. 7. In vitro E3 activity and selfubiquitination in BBAP/DTX1 heterodimers. Myc-tagged wild-type DTX1-GST protein was added to FLAG-tagged wild-type BBAP-GST or BBAP RFdel -GST protein; wild-type BBAP protein was also incubated separately. After incubation with the additional required components (E1, E2, and biotinylated ubiquitin), samples were size-fractionated and serially Western-immunoblotted (WB) with anti-FLAG (BBAP) antibody (left panel) and peroxidase-conjugated avidin (biotinylated ubiquitin) (right panel).
Nevertheless, previous murine studies indicate that DTX1, DTX2, and DTX3 form homo-and heterodimers (6). Recently, heterodimeric RING finger protein complexes have been associated with increased functional activity. For example, the heterodimeric complex containing the BRCA1 tumor suppressor and BARD1 exhibits increased E3 activity (29,30). Similarly, the complex containing two different negative regulators of p53, Mdm2 and MdmX, has increased E3 activity (31). That BBAP/DTX1 heterodimers exhibit increased E3 activity is consistent with these observations. However, unlike the Mdm2 and MdmX proteins, which interact via their respective RING finger domains (32), BBAP binds to DTX1 via its unique N terminus. In our studies, a C-terminal BBAP construct containing the RING finger did not bind full-length DTX1 or BBAP. Therefore, other protein interaction domains could be responsible for the heterodimerization of certain RING finger-containing proteins and the consequent increase in E3 activity.
Because BBAP also interacts with the BAL protein via its unique N terminus, the functional consequences of this interaction are of great interest. Although BBAP is clearly an E3, BAL has not yet been identified as a substrate for BBAPmediated ubiquitination.
Given the relevance of the DTX family of proteins in the control of several critical cellular mechanisms, the demonstration of their E3 activity is important. This newly identified function, as well as the role of DTX1 in modulating BBAP E3 activity, should be taken in consideration when studying the effects of these proteins in vivo.