Expression of MYB proteins in avian hematopoietic tissues

MYB gene products are thought to be regulators of cellular replication and of differentiation. The major product of the avian MYB gene is a 75 kDa nuclear phosphoprotein which can activate transcription. A minor 89 kDa MYB protein of unknown function has also been described in murine and human cells. Additional heterogeneity at the level of MYB RNA which could affect the structure of MYB proteins has been described in several species. Such heterogeneity could explain the diverse effects of the MYB gene. To investigate the possible existence of heterogeneous and/or cell lineage-specific MYB proteins, five different avian hematopoietic tissues (bone marrow, bursa of Fabricius, embryonic spleen, thymus and yolk sac) were examined by immunoprecipitation with several MYB-specific antisera and SDS-PAGE analysis. In all five tissues there was a 75 kDa protein of uniform size which varied in abundance in a tissue-specific manner paralleling that observed for the 4.0 kb MYB RNA. A less abundant 89 kDa protein was also detected by several antisera in bone marrow, spleen, thymus and yolk sac but not in bursa. This 89 kDa MYB protein appears to be analogous to the 89 kDa MYB protein encoded by a minor but larger (360 nucleotides) MYB mRNA in murine and human cells. Immunoprecipitation of MYB proteins with an antiserum specific for exons 8 and 9 revealed a 74 kDa protein which co-precipitated and appeared to be complexed with p75 in normal hematopoietic cells and with the 48 kDa product of v-myb in leukemic cells.