The SCL-LMO2 Interaction Nucleates Hematopoietic Transcription Factor Complexes: Coupling Protein Stability and Complex Assembly.
Bibliographic record
Abstract
Abstract The differentiation of hematopoietic cells is tightly controlled by transcription factor complexes, composed of hemato-specific and ubiquitous proteins. The bHLH factor SCL and the LIM-only protein LMO2 are central components of transcription factor complexes and are essential for hematopoiesis. However, the mechanism regulating the assembly of SCL-complexes is unknown. Here we show that SCL, in contrast to LMO, GATA and E proteins, cannot be replaced by other members of its family in hematopoietic gene transactivation and in gel shift assays. Furthermore, we show by GST pull-down assays and by co-immunoprecipitation that interaction with LMO2 is a unique property of SCL, as the neurogenic bHLH NSCL1 related to SCL cannot bind LMO2. By generating SCL-NSCL1 chimeras, and by phylogenetic alignment, we identified the SCL interface that confers transcriptional specificity to the complex. Strikingly, this interface is also necessary for the interaction with LMO2. In contrast with the wild type SCL, the mutant without this interface is not able to enhance erythroid differentiation when overexpressed in hematopoietic cells, as assessed by glycophorin A gene activation, benzidine staining and methylcellulose cultures. Interestingly, we also demonstrate in vivo and in vitro that LMO2 protein levels are greatly increased in the presence of SCL, while mRNA levels remain constant. When the SCL interface described above is mutated, LMO2 protein level is no longer increased, suggesting that the accumulation of LMO2 is mediated by a direct interaction with SCL. In primary hematopoietic cells, when SCL protein levels are genetically reduced by LacZ insertion into one allele in the SCL locus, we observe a dramatic decrease in LMO2 protein levels. In addition, in the TF-1 hematopoietic cell line, most of the LMO2 protein (90%) is found associated with SCL and/or Ldb1, suggesting that free LMO2 is rapidly degraded. Thus, SCL levels determine LMO2 levels in hematopoietic cells. Next, we provide direct evidence that LMO2 is a target for proteasomal degradation. First, we show that LMO2 is ubiquitinated in vivo, by GST purification. Second, by using the ts20 cell line expressing a temperature-sensitive ubiquitin-activating E1 enzyme, we show that LMO2 degradation requires a functional ubiquitin conjugation system, since LMO2 is not degraded when E1 is inactive. Third, we show that the half-life of LMO2 is very short, and it can be increased with the proteasome inhibitor MG132. Finally, a similar increase in LMO2 half-life can be observed when SCL is co-expressed with LMO2. These data indicate that SCL stabilizes LMO2, which is otherwise rapidly degraded by the ubiquitin-proteasome pathway in absence of its interacting partners. Taken together, our results strongly suggest that SCL, by binding and stabilizing LMO2, is a critical determinant of the hematopoietic transcriptional specificity. The interaction between SCL and LMO2 is an essential nucleation step for the assembly of SCL-complexes on DNA, where the regulation of LMO2 levels appears to be the rate-limiting step. We propose that protein stability is a new mechanism of regulation in the formation of SCL complexes, required for proper gene activation during eryhtroid differentiation.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".