The LIM-only protein Lmo2, activated by chromosomal translocations in T-cell leukaemias, is normally expressed in haematopoiesis. It interacts with TAL1 and GATA-1 proteins, but the function of the interaction is unexplained. We now show that in erythroid cells Lmo2 forms a novel DNA-binding complex, with GATA-1, TAL1 and E2A, and the recently identified LIM-binding protein Ldb1/NLI. This oligomeric complex binds to a unique, bipartite DNA motif comprising an E-box, CAGGTG, followed approximately 9 bp downstream by a GATA site. In vivo assembly of the DNA-binding complex requires interaction of all five proteins and establishes a transcriptional transactivating complex. These data demonstrate one function for the LIM-binding protein Ldb1 and establish a function for the LIM-only protein Lmo2 as an obligatory component of an oligomeric, DNA-binding complex which may play a role in haematopoiesis.

The LMO2 and TAL1 genes were first identified via chromosomal translocations and later found to encode proteins that interact during normal erythroid development. Some T cell leukaemia patients have chromosomal abnormalities involving both genes, implying that LMO2 and TAL1 act synergistically to promote tumorigenesis after their inappropriate co‐expression. To test this hypothesis, transgenic mice were made which co‐express Lmo2 and Tal1 genes in T cells. Dimers of Lmo2 and Tal1 proteins were formed in thymocytes of double but not single transgenic mice. Furthermore, thymuses of double transgenic mice were almost completely populated by immature T cells from birth, and these mice develop T cell tumours approximately 3 months earlier than those with only the Lmo2 transgene. Thus interaction between these two proteins can alter T cell development and potentiate tumorigenesis. The data also provide formal proof that TAL1 is an oncogene, apparently acting as a tumour promoter in this system.

The protein products of proto‐oncogenes implicated in T cell acute lymphoblastic leukemia include two distinct families of presumptive transcription factors. RBTN1 and RBTN2 encode highly related proteins that possess cysteine‐rich LIM motifs. TAL1, TAL2 and LYL1 encode a unique subgroup of basic helix‐loop‐helix (bHLH) proteins that share exceptional homology in their bHLH sequences. We have found that RBTN1 and RBTN2 have the ability to interact with each of the leukemogenic bHLH proteins (TAL1, TAL2 and LYL1). These interactions occur in vivo and appear to be mediated by sequences within the LIM and bHLH domains. The LIM‐bHLH interactions are highly specific in that RBTN1 and RBTN2 will associate with TAL1, TAL2 and LYL1, but not with other bHLH proteins, including E12, E47, Id1, NHLH1, AP4, MAX, MYC and MyoD1. Moreover, RBTN1 and RBTN2 can interact with TAL1 polypeptides that exist in assembled bHLH heterodimers (e.g. TAL1‐E47), suggesting that the RBTN proteins can influence the functional properties of TAL1. Finally, we have identified a subset of leukemia patients that harbor tumor‐specific rearrangements of both their RBTN2 and TAL1 genes. Thus, the activated alleles of these genes may promote leukemia cooperatively, perhaps as a result of bHLH‐LIM interactions between their protein products.

Tumor-specific activation of the TALI gene occurs in "r.25% of patients with T-cell acute lymphoblastic leukemia (T-ALL). The TALI gene products possess a basic helix-oop-belix (bHLH) domain that interacts in vitro with the bHLH proteins (E12 and E47) encoded by the E2A locus. We have now applied two independent methods, the two-hybrid procedure and co-immunoprecipitation analysis, to demonstrate that TALl and E2A polypeptides also associate in vivo.These studies show that the bHLH domain of TALl selectively interacts with the bHLH domains of E12 and E47, but not with the Idl helix-oop-helix protein. TALl does not self-asiate to form homodimeric complexes, implying that the in vivo functions of TALl depend on heterologous interaction with other bHLH proteins such as E12 and E47. Co-immunoprecipitation analysis revealed the presence of endogenous TAL1/E2A complexes in Jurkat cells, a leukemic line derived from a T-ALL patient. Thus, the malignant properties ofTALl may be due to obligate interaction with the E2A polypeptides.

Tumor-specific activation of the TALI gene is the most common genetic defect acted with T-cell acute lymphoblatic leukemia. The TALI gene products possess a basic helix-oop-helix (bHLH)