Role of myb gene family in the proliferation control of hematopoietic cells

到目前为止所获得的结果都表明c-myb在维持造血祖细胞的增殖状态中起作用。通过定点突变破坏这种亮氨酸拉链，显著增加了反式激活和转化能力。由于亮氨酸拉链基序最初被鉴定为介导几种脱氧核糖核酸结合蛋白的二聚化，这些结果表明c-Myb活性通过亮氨酸拉链受到负调控，并暗示c-Myb抑制剂(S)的存在。通过胚胎干细胞同源重组产生的纯合子c-myb突变小鼠由于胎儿肝脏造血的特定失败而在胚胎阶段死亡。因此，该突变体不能用于成人组织中c-myb功能的分析。我们尝试在ES细胞中进行两步同源重组，获得c-myb点突变小鼠。在这些点突变中，c-Myb活性预计是野生型的1/10或10倍，突变小鼠在胚胎阶段不会致死。我们已经制作了杂合点突变，因此预期的纯合点突变将对MYB功能的分析非常有用。我们发现c-Myb以不依赖于磷酸化的方式与CBP结合。由于CBP与TFIIB相互作用，CBP介导依赖于CREB或c-Myb的转录激活，作为磷酸化的CREB或c-Myb与RNA聚合酶II复合体之间的桥梁。最近，CBP被证明是包括c-jun和c-Fos在内的许多其他转录激活因子的转录辅助激活因子。有趣的是，人类CBP基因的突变通过单倍体不足导致Rubinstein-Taybi综合征，这表明细胞中CBP的数量并不过量，CBP数量减少50%会影响正常发育。因此，通过竞争结合CBP，c-Myb、c-Myb、cAMP和AP-1通路之间的串扰是可能的。

The reults obtained so far all indicate a role for c-myb in maintaining the proliferative state of hematopoietic prgenitor cells. Disruption of this leucine zipper by site-directed mutagenesis markedly increases both trans-activating and transforming capacities. Since the leucine zipper motif was originallyn identified as mediating dimerization of several DNA-binding proteins, these results indicate that c-Myb activity is negatively regulated through the leucine zipper and imply the presence of an inhibitor (s) of c-Myb.Homozygous c-myb mutant mice generated by homologous recombination in embryonic stem cells die at the embryonic stage due to a specific failure of fetal hepatic haematopoiesis. Therefore, this mutant cannot used for the analyzes of c-myb function in the adult tissues. We have tried to make the point mutant mice of c-myb by two step homologous recombination in ES cells. In these point mutants, the c-Myb activity is expected to be 1/10 or 10-fold compared with the wild type, and the mutant mice is not lethal at embryonic stage. We have made the heterozygous point mutants, so that the expected homozgous mutants will be very useful for the analyzes of myb function.We have found that c-Myb binds to CBP in a phosphorylation-independent manner. Since CBP interacts with TFIIB,CBP mediates CREB-or c-Myb-dependent transcriptional activation as a bridge between phosphorylated CREB or c-Myb and the RNA polymerase II complex. Recently, CBP was demonstated to function as a transcriptional coactivator for many other transcriptional activators including c-Jun and c-Fos. Interestingly, mutations in the human CBP gene cause Rubinstein-Taybi syndrome through haploinsufficiency, suggesting that theamount of CBP in cells is not excessive and a 50% decrease in the amount affects normal development. Therefore, cross talk between the c-Myb, c-Myb, cAMP,and AP-1 pathways through competition for binding to CBP is possible.