SCL Assembles a Multifactorial Complex That Determines Glycophorin A Expression

Abstract

SCL/TAL1 is a hematopoietic-specific transcription factor of the basic helix-loop-helix (bHLH) family that is essential for erythropoiesis. Here we identify the erythroid cell-specific glycophorin A gene (GPA) as a target of SCL in primary hematopoietic cells and show that SCL occupies the GPA locus in vivo. GPA promoter activation is dependent on the assembly of a multifactorial complex containing SCL as well as ubiquitous (E47, Sp1, and Ldb1) and tissue-specific (LMO2 and GATA-1) transcription factors. In addition, our observations suggest functional specialization within this complex, as SCL provides its HLH protein interaction motif, GATA-1 exerts a DNA-tethering function through its binding to a critical GATA element in the GPA promoter, and E47 requires its N-terminal moiety (most likely entailing a transactivation function). Finally, endogenous GPA expression is disrupted in hematopoietic cells through the dominant-inhibitory effect of a truncated form of E47 (E47-bHLH) on E-protein activity or of FOG (Friend of GATA) on GATA activity or when LMO2 or Ldb-1 protein levels are decreased. Together, these observations reveal the functional complementarities of transcription factors within the SCL complex and the essential role of SCL as a nucleation factor within a higher-order complex required to activate gene GPA expression.

Rachid Lahlil and Eric Lécuyer contributed equally to this work.

We thank Peter D. Aplan for SIL-SCL transgenic mice as well as Stuart H. Orkin, Catherine Porcher, John D. Crispino, and Jacques Drouin for providing expression vectors for SCL mutants, the GATA-1 V205→R mutant, FOG, and E47-bHLH, respectively.

This work was supported in part by a grant from the Canadian Institute for Health Research (CIHR) to T.H.; by a studentship from the CIHR (E.L.), and by a fellowship from the Leukemia Research Fund of Canada (S.H.).