T-cell oncogene rhombotin-2 interacts with retinoblastoma-binding protein 2

The LIM domain protein rhombotin-2 (RBTN-2/TTG-2/Lmo2) has distinct functions in erythropoiesis and in T-cell leukemogenesis. Additional functions for RBTN2 are indicated by its expression in non-hematopoietic tissues. These diverse functions of RBTN2 are presumed to be accomplished through physical interaction with different protein partners that bind the LIM domains of RBTN2. To identify these proteins which may modulate the activity of RBTN2, a human cDNA library was screened using the yeast two-hybrid assay. Using the RBTN2 LIM domain region as `bait', the retinoblastoma-binding protein 2 (RBP2) was identified as a partner for RBTN2. The interaction between RBTN2 and RBP2 was confirmed using in vitro binding assays, and by co-immunoprecipitation of the two proteins. Deletion analysis showed the second LIM domain of RBTN2 was necessary and sufficient for binding to the last 69 amino acids of RBP2. The interaction between RBTN2 and RBP2 had a functional consequence: the combination of RBP2 and RBTN2 gave higher transcription in vitro, than RBTN2 alone. The interaction with RBP2 suggests two additional functions for RBTN2: (i) RBTN2 may directly affect the activity of RBP2, and/or (ii) RBTN2 may indirectly modulate the functions of the retinoblastoma protein by binding to RBP2.