Serine Phosphorylation On TAL1 Regulates Its Interaction with Histone Demethylase LSD1.

Abstract 1460

Poster Board I-483

TAL1, originally identified by virtue of its involvement in a T-cell acute lymphoblastic leukemia (T-ALL)-specific chromosomal translocation, is a member of the basic helix-loop-helix (bHLH) family of transcription factors and is required for the development of all hematopoietic cell lineages. TAL1 is a phosphorylated protein and its activities are mediated by the corepressors and coactivators that associate with TAL1. However, the functional link between phosphorylation and the recruitment of co-regulators by TAL1 is currently unknown. We undertook the biochemical purification of the TAL1 containing complexes and showed that TAL1 is associated with histone demethylase complexes containing LSD1, CoREST, HDAC1 and HDAC2. This complex mediates TAL1 directed transcriptional repression during hematopoiesis. The interaction between TAL1 and LSD1 are dynamically regulated and is required for TAL1's function in erythroid differentiation (Proc. Natl. Acad. Sci. USA 106: 10141-10146). To further understand the molecular mechanism that regulates the TAL1 and LSD1 interaction during hematopoiesis, we determined whether TAL1 directly interacts with LSD1 and characterized the domains required for this interaction. TAL1 and its various deletion mutants were tested for their ability to interact with LSD1 in vitro. TAL1 directly interacts with LSD1, and the interacting domain encompasses amino acids 142-185 proximal to the bHLH domain, which contains a serine 172 residue that becomes phosphorylated during transcriptional activation of TAL1. We further mutated serine 172 of TAL1 to Alanine (Ala) or to Aspartic acid (Asp) to mimic unphosphorylated or phosphorylated TAL1, respectively. While the TAL1^Ser172Ala mutant remains the interaction with LSD1, TAL1^Ser172Asp specifically loses its interaction with LSD1 indicating that serine 172 phosphorylation of TAL1 destabilizes the TAL1 and LSD1 interaction. Given that our previous results indicated that LSD1 inhibits TAL1 mediated erythroid differentiation, to further test whether the activity of TAL1 is mediated through interaction with LSD1, we expressed the TAL1 mutant that deleted the LSD1 interacting domain in erythroid progenitor cells and showed that the deletion of the LSD1 interacting domain of TAL1 lead to a promotion of erythroid differentiation and inhibition of proliferation. Furthermore, consistent with the rapid decline of TAL1-associated LSD1 complex during differentiation, the ChIP and ChIP-seq data showed that H3K4 di- and tri-methylation are enriched at the promoters of TAL1 target genes upon erythroid differentiation. Thus, our data revealed that histone lysine demethylase LSD1 may negatively regulate TAL1-mediated transcription and erythroid differentiation. The results suggest that the dynamic regulation of TAL1-associated LSD1/HDAC1 complex may determine the onset of erythroid differentiation programs.

* These authors contribute equally to this work.