v-Myb DNA binding is required to block thrombocytic differentiation of Myb-Ets-transformed multipotent haematopoietic progenitors.

The E26 avian leukaemia virus encodes a fusion oncoprotein consisting of truncated versions of the c-Myb and c-Ets-1 transcription factors. When used to infect embryonic chicken haematopoietic cells two types of self-renewing progenitors are obtained, namely myeloblasts and 'MEPs' (Myb-Ets progenitors). In earlier work we have shown that myeloblasts transformed by the ts21 mutant of E26, which has a lesion in v-Myb, can be induced to differentiate into macrophages following shift to the non-permissive temperature. Here we show that the ts21 v-Myb is temperature sensitive for DNA binding in band shift experiments and that its inactivation in transformed MEPs induces their maturation into thrombocytes. The MEP transforming capacity of v-Myb is not confined to its fusion with v-Ets, as it is also seen with a virus that co-expresses tsMyb with v-ErbB. As with wild-type E26-transformed MEPs, ts21-transformed MEPs are multipotent, differentiating into eosinophils and myeloblasts following treatment with 12-O-tetradecanoylphorbol-13-acetate. In addition, ts21-transformed myeloblasts differentiate into macrophages when shifted to the non-permissive temperature. This shows that v-Myb blocks haematopoietic differentiation at two distinct stages. In contrast, v-Ets inactivation in MEPs transformed by a ts E26 mutant with a lesion in the corresponding oncoprotein leads to their differentiation into erythrocytes, myeloblasts and probably eosinophils. These data show that the two domains of Myb-Ets selectively affect decision making processes in different types and stages of haematopoietic cells.