AU-rich elements (AREs) and microRNA target sites are conserved sequences in messenger RNA (mRNA) 3' untranslated regions (3'UTRs) that control gene expression posttranscriptionally. Upon cell cycle arrest, the ARE in tumor necrosis factor-alpha (TNFalpha) mRNA is transformed into a translation activation signal, recruiting Argonaute (AGO) and fragile X mental retardation-related protein 1 (FXR1), factors associated with micro-ribonucleoproteins (microRNPs). We show that human microRNA miR369-3 directs association of these proteins with the AREs to activate translation. Furthermore, we document that two well-studied microRNAs-Let-7 and the synthetic microRNA miRcxcr4-likewise induce translation up-regulation of target mRNAs on cell cycle arrest, yet they repress translation in proliferating cells. Thus, activation is a common function of microRNPs on cell cycle arrest. We propose that translation regulation by microRNPs oscillates between repression and activation during the cell cycle.

MicroRNAs are small regulatory RNA molecules that exert post-transcriptional control overexpression of specific target mRNAs. AU-rich elements (AREs) are highly conserved 3'UTR sequences that alter the stability and translation of mRNAs of clinical importance as a rapid and transient response to external and internal changes. We recently demonstrated that a reporter mRNA containing the tumor necrosis factor alpha (TNFalpha) ARE activates translation in response to quiescence via microRNA target sites in the ARE. Further studies revealed that microRNAs in general have the potential to regulate translation in a cell cycle determined manner: in quiescent cells, microRNAs activate translation while in cycling/proliferating cells, microRNAs repress translation.

DET1 (de-etiolated 1) is an essential negative regulator of plant light responses, and it is a component of theArabidopsis thaliana CDD complex containing DDB1 and COP10 ubiquitin E2 variant. Human DET1 has recently been isolated as one of the DDB1-and Cul4A-associated factors, along with an array of WD40-containing substrate receptors of the Cul4A-DDB1 ubiquitin ligase. However, DET1 differs from conventional substrate receptors of cullin E3 ligases in both biochemical behavior and activity. Here we report that mammalian DET1 forms stable DDD-E2 complexes, consisting of DDB1, DDA1 (DET1, DDB1 associated 1), and a member of the UBE2E group of canonical ubiquitin-conjugating enzymes. DDD-E2 complexes interact with multiple ubiquitin E3 ligases. We show that the E2 component cannot maintain the ubiquitin thioester linkage once bound to the DDD core, rendering mammalian DDD-E2 equivalent to the Arabidopsis CDD complex. While free UBE2E-3 is active and able to enhance UbcH5/Cul4A activity, the DDD core specifically inhibits Cul4A-dependent polyubiquitin chain assembly in vitro. Overexpression of DET1 inhibits UV-induced CDT1 degradation in cultured cells. These findings demonstrate that the conserved DET1 complex modulates Cul4A functions by a novel mechanism.The ubiquitin conjugation system is used as a common regulatory strategy in all eukaryotic organisms (12). Ubiquitin is activated by the E1 ubiquitin-activating enzyme. It is then transferred to the catalytic site cysteine residue in a ubiquitinconjugating enzyme (E2), forming an E2ϳubiquitin thioester intermediate. Finally, the ubiquitin-charged E2 cooperates with an E3 ligase to attach ubiquitin to a substrate protein. For those substrates targeted to the 26S proteasome, the initial ubiquitin attachment is followed by chain elongation (14).DET1 (de-etiolated 1) was originally isolated as a key regulator of light-activated development in Arabidopsis thaliana (6) and in tomato (known as HP2 for high pigment 2) (26). Det1 mutants have an altered gene expression pattern and are unable to undergo the etiolation developmental path in darkness (6,24,28,33). In addition, the weak mutants are hypersensitive to light signals and exhibit an abnormal circadian rhythm. DET1 has been shown to negatively regulate ubiquitin-proteasome-mediated turnover of an important circadian clock regulator, LHY (late elongated hypocotyl) (34). DET1 forms a protein complex known as the CDD complex with the Arabidopsis homolog of DDB1 (damaged DNA binding protein 1) (33) and COP10 (41). COP10 is a ubiquitin E2 variant (UEV) that has the conserved catalytic core domain (UBC) but lacks the cysteine residue that forms the thioester linkage with ubiquitin and is therefore catalytically inactive (36). COP10 is distinct from other known UEVs, such as MMS2, UEV1, and TSG101, as it displays highest homology to the Saccharomyces cerevisiae Ubc4/5 family of canonical E2s (20, 36). The Arabidopsis COP10 belongs to the COP/DET/FUS class of loci that also includes conserved genes such as DET1, COP1, an...

Transcription factors (TFs) play multiple roles in development. Given this multifunctionality, it has been assumed that TFs are evolutionarily highly constrained. Here, we investigate the molecular mechanisms for the origin of a derived functional interaction between two TFs, HOXA11 and FOXO1. We have previously shown that the regulatory role of HOXA11 in mammalian endometrial stromal cells requires interaction with FOXO1, and that the physical interaction between these proteins evolved before their functional cooperativity. Here, we demonstrate that the derived functional cooperativity between HOXA11 and FOXO1 is due to derived allosteric regulation of HOXA11 by FOXO1. This study shows that TF function can evolve through changes affecting the functional output of a pre-existing protein complex.