HELIX-LOOP-HELIX PROTEINS LYL1 AND E2A FORM HETERODIMERIC COMPLEXES WITH DISTINCTIVE DNA-BINDING PROPERTIES IN HEMATOLYMPHOID CELLS

LYL1 is a basic helix-loop-helix (HLH) protein that was originally dis covered because of its translocation into the beta T-cell receptor loc us in an acute lymphoblastic leukemia, LYL1 is expressed in many hemat olymphoid cells, with the notable exceptions of thymocytes and T cells , Using the yeast two-hybrid system to screen a cDNA library construct ed from B cells, we identified the E-box-binding proteins E12 and E47 as potential lymphoid dimerization partners for LYL1, The interaction of LYL1 with E2a proteins was further characterized in vitro and shown to require the HLH motifs of both proteins, Immunoprecipitation analy ses showed that in T-ALL and other cell lines, endogenous LYL1 exists in a complex with E2a proteins. A preferred DNA-binding sequence, 5'-A ACAGATG(T/g)T-3', for the LYL1-E2a heterodimer was determined by PCR-a ssisted site selection. Endogenous protein complexes containing both L YL1 and E2a bound this sequence in various LYL1-expressing cell lines and could distinguish between the LYL1 consensus and mu E2 sites, Thes e data demonstrate that E2a proteins serve as dimerization partners fo r the basic HLH protein LYL1 to form complexes with distinctive DNA-bi ndiug properties and support the hypothesis that the leukemic properti es of the LYL and TAL subfamily of HLH proteins could be mediated by r ecognition of a common set of target genes as heterodimeric complexes with class I HLH proteins.