Decoding hematopoietic specificity in the helix-loop-helix domain of the transcription factor SCL/Tal-1

The helix-loop-helix (HLH) domain is employed by many transcription factors that control cell fate choice in multiple developmental settings. Previously, we demonstrated that the HLH domain of the class II basic HLH (bHLH) protein SCL/Tal-1 is critical for hematopoietic specification. We have now identified residues in this domain that are essential for restoring hematopoietic development to SCL-/- embryonic stem cells and sufficient to convert a muscle-specific HLH domain to one able to rescue hematopoiesis. Most of these critical residues are distributed in the loop of SCL, with one in helix 2. This is in contrast to the case for MyoD, the prototype of class II bHLH proteins, where the loop seems to serve mainly as a linker between the two helices. Among the identified residues, some promote heterodimerization with the bHLH partners of SCL (E12/E47), while others, unimportant for this property, are still crucial for the biological function of SCL. Importantly, the residue in helix 2 specifically promotes interaction with a known partner of SCL, the LIM-only protein LMO2, a finding that strengthens genetic evidence that these proteins interact. Our data highlight the functional complexity of bHLH proteins, provide mechanistic insight into SCL function, and strongly support the existence of an active SCL/LMO2-containing multiprotein complex in early hematopoietic cells.