Chromosomal translocations involving the MLL gene are associated with infant acute lymphoblastic and mixed lineage leukemia. There are a large number of translocation partners of MLL that share very little sequence or seemingly functional similarities, however, their translocations into MLL result in the pathogenesis of leukemia. To define the molecular reason why these translocations result in the pathogenesis of leukemia, we purified several of the commonly occurring MLL chimeras. We have identified a novel super elongation complex (SEC) associated with all chimeras purified. SEC includes ELL, P-TEFb, AFF4 and several other factors. AFF4 is required for SEC stability and proper transcription by poised RNA polymerase II in metazoans. Knockdown of AFF4 within SEC in leukemic cells shows reduction in MLL chimera target gene expression suggesting that AFF4/SEC could be a key regulator in the pathogenesis of leukemia through many of the MLL partners.

Summary Promoter proximal pausing by initiated RNA polymerase II (Pol II) and regulated release of paused polymerase into productive elongation has emerged as a major mechanism of transcription activation. Reactivation of paused Pol II correlates with recruitment of SuperElongationComplexes (SECs) containing ELL/EAF family members, P-TEFb, and other proteins, but the mechanism of their recruitment is currently a major unanswered question. Here, we present evidence for a role of human Mediator subunit Med26 in this process. We identify in the conserved N-terminal domain of Med26 overlapping docking sites for SEC and a second ELL/EAF-containing complex, as well as general initiation factor TFIID. In addition, we present evidence consistent with the model that Med26 can function as a molecular switch that interacts first with TFIID in the Pol II initiation complex and then exchanges TFIID for complexes containing ELL/EAF and P-TEFb to facilitate transition of Pol II into the elongation stage of transcription.

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SUMMARY Deubiquitinating enzymes (DUBs) are proteases that can antagonize ubiquitin-mediated signalling by disassembling ubiquitin-protein conjugates. How DUBs are regulated in vivo and how their substrate specificities are achieved are largely unknown. The conserved DUB Uch37 is found on proteasomes in organisms ranging from fission yeast to humans. Deubiquitination by Uch37 is activated by proteasomal binding, which enables Uch37 to process polyubiquitin chains. Here we show that in the nucleus Uch37 is also associated with the human Ino80 chromatin-remodeling complex (hINO80). In hINO80, Uch37 is held in an inactive state; however, it can be activated by transient interaction of the Ino80 complex with the proteasome. Thus, DUB activities can be modulated both positively and negatively via dynamic interactions with partner proteins. In addition, our findings suggest that the proteasome and the hINO80 chromatin-remodeling complex may cooperate to regulate transcription or DNA repair, processes in which both complexes have been implicated.

SUMO-1 is a member of a family of ubiquitin-like molecules that are post-translationally conjugated to various cellular proteins in a process that is mechanistically similar to ubiquitylation. To identify molecules that bind noncovalently to SUMO-1, we performed yeast twohybrid screening with a SUMO-1 mutant that cannot be conjugated to target proteins as the bait. This screening resulted in the isolation of cDNAs encoding the b isoform of thymine DNA glycosylase (TDGb). A deletion mutant of TDGb (TDGb(⌬11)) that lacks a region shown to be required for noncovalent binding of SUMO-1 was also found not to be susceptible to SUMO-1 conjugation at an adjacent lysine residue, suggesting that such binding is required for covalent modification. In contrast, another mutant of TDGb (TDGb(KR)) in which the lysine residue targeted for SUMO-1 conjugation is replaced with arginine retained the ability to bind SUMO-1 noncovalently. TDGb was shown to interact with the promyelocytic leukemia protein (PML) in vitro as well as to colocalize with this protein to nuclear bodies in transfected cells. TDGb(KR) also colocalized with PML, whereas TDGb(⌬11) did not, indicating that the noncovalent SUMO-1 binding activity of TDGb is required for colocalization with PML. Furthermore, SUMO-1 modification of TDGb and PML enhanced the interaction between the two proteins. These results suggest that SUMO-1 functions to tether proteins to PML-containing nuclear bodies through post-translational modification and noncovalent protein-protein interaction.Post-translational modification of proteins plays important roles in the regulation of protein function and localization. Proteins are chemically modified by various molecules, including phosphate, lipids, and sugars. Modification by ubiquitin is distinct in that the modifier itself is a small protein. Ubiquitin is usually attached to a lysine residue of target proteins, resulting in the formation of a branched isopeptide chain. Such ubiquitylation serves to mark proteins for degradation by the 26 S proteasome.SUMO-1 is a member of the ubiquitin-like protein superfamily and is post-translationally conjugated to various cellular proteins in a process that is mechanistically analogous to ubiquitylation. SUMO-1 modification is mediated by a SUMO-1-activating enzyme (E1), 1 a SUMO-1-conjugating enzyme (E2), and a SUMO-1 ligase (E3) (1, 2). SUMO-1 is attached to target proteins via an isopeptide bond between the COOH-terminal glycine residue of SUMO-1 and the ⑀-amino group of a target lysine residue. A single E1 enzyme (the Aos1-Uba2 heterodimer) and a single E2 enzyme (Ubc9) have been identified for the SUMO-1 modification pathway in yeast and higher eukaryotes (2, 3). These two enzymes are sufficient to modify various SUMO-1 targets, including IB␣ (4), RanGAP1 (5), and p53 (6) in vitro, and it had been thought that SUMO-1 modification does not require an E3 ligase. However, several E3-like factors (PIAS family, RanBP2, PC2) for SUMO-1 modification were recently identified in yeast and mammalian cells ...