Association of MTG8 (ETO/CDR), a Leukemia‐related Protein, with Serine/Threonine Protein Kinases and Heat Shock Protein HSP90 in Human Hematopoietic Cell Lines

A proto‐oncogene, MTG8 (ETO/CDR), is disrupted in the t(8;21) translocation associated with acute myeloid leukemia, and the gene product, MTG8, is a phosphoprotein capable of cell transformation in concert with v‐H‐ras. To obtain insight into functional regulation of MTG8 by phosphorylation, we studied protein kinases that interact with, and phosphorylate, MTG8 in vitro. Recombinant MTG8 protein was first found to be associated with two serine/threonine protein kinases in cell extracts from both HEL cells and a leukemic cell line carrying t(8;21). A cytoplasmic protein kinase of 61 kDa (MTG8N‐kinase) phosphorylated the amino‐terminal of MTG8, and another of 52 kDa (MTG8C‐kinase) phosphorylated the carboxyl‐terminal domain. In addition, we demonstrated that heat shock protein 90 (HSP90) specifically binds to the amino‐terminal domain of MTG8 in vitro and in vivo. Thus, our results shed new light on post‐translational regulation of MTG8, perturbation of which, in AML1‐MTG8 protein, probably contributes to leukemogenesis.