Brother and Big brother were isolated as Runt-interacting
proteins and are homologous to CBF(beta), which interacts with the mammalian
CBF(alpha) Runt-domain
proteins. In vitro experiments indicate that Brother family
proteins regulate the
DNA binding activity of Runt-domain
proteins without contacting
DNA. In both mouse and human there is genetic evidence that the
CBF(alpha) and CBF(beta)
proteins function together in hematopoiesis and leukemogenesis. Here we demonstrate functional interactions between Brother
proteins and Runt domain
proteins in Drosophila. First, we show that a specific point mutation in Runt that disrupts interaction with Brother
proteins but does not affect
DNA binding activity is dysfunctional in several in vivo assays. Interestingly, this
mutant protein acts dominantly to interfere with the Runt-dependent activation of Sxl-lethal transcription. To investigate further the requirements for Brother
proteins in Drosophila development, we examine the effects of expression of a Brother fusion
protein homologous to the dominant negative
CBF(beta)::SMMHC fusion protein that is associated with
leukemia in humans. This Bro::SMMHC fusion
protein interferes with the activity of Runt and a second Runt domain protein, Lozenge. Moreover, we find that the effects of lozenge mutations on eye development are suppressed by expression of wild-type Brother
proteins, suggesting that Brother/Big brother dosage is limiting in this developmental context. Results obtained when Runt is expressed in developing eye discs further support this hypothesis. Our results firmly establish the importance of the Brother and Big brother
proteins for the
biological activities of Runt and Lozenge, and further suggest that Brother
protein function is not restricted to enhancing
DNA-binding.