Exogenous cdk4 overcomes reduced cdk4 RNA and inhibition of G1 progression in hematopoietic cells expressing a dominant-negative CBF - a model for overcoming inhibition of proliferation by CBF oncoproteins.

Core Binding Factor (CBF) is required for the development of definitive hematopoiesis, and the CBF oncoproteins AML1-ETO, TEL-AML1, and CBFbeta-SMMHC are commonly expressed in subsets of acute leukemia. CBFbeta-SMMHC slows the G1 to S cell cycle transition in hematopoietic cells, but the mechanism of this effect is uncertain. We have sought to determine whether inhibition of CBF-mediated trans-activation is sufficient to slow proliferation. We demonstrate that activation of KRAB-AML1-ER, a protein containing the AML1 DNA-binding domain, the KRAB repression domain, and the Estrogen receptor ligand binding domain, also slows G1, if its DNA-binding domain is intact. Also, exogenous AML1 overcame CBFbeta-SMMHC-induced inhibition of proliferation. Representational difference analysis (RDA) identified cdk4 RNA expression as an early target of KRAB-AML1 activation. Inhibition of CBF activities by KRAB-AML1-ER or CBFbeta-SMMHC rapidly reduced endogenous cdk4 mRNA levels, even in cells proliferating at or near control rates as a result of exogenous cdk4 expression. Over-expression of cdk4, especially a variant which cannot bind p16INK4a, overcame cell cycle inhibition resulting from activation of KRAB-AML1-ER, although cdk4 did not accelerate proliferation when expressed alone. These findings indicate that mutations which alter the expression of G1 regulatory proteins can overcome inhibition of proliferation by CBF oncoproteins. Oncogene (2000).

Solution structure of the PHD domain from the KAP-1 corepressor: structural determinants for PHD, RING and LIM zinc-binding domains.

Plant homeodomain (PHD) domains are found in >400 eukaryotic proteins, many of which are transcriptional regulators. Naturally occurring point mutations or deletions of this domain contribute to a variety of human diseases, including ATRX syndrome, myeloid leukemias and autoimmune dysfunction. Here we report the first structural characterization of a PHD domain. Our studies reveal that the PHD domain from KAP-1 corepressor binds zinc in a cross-brace topology between anti-parallel ss-strands reminiscent of RING (really interesting new gene) domains. Using a mutational analysis, we define the structural features required for transcriptional repression by KAP-1 and explain naturally occurring, disease-causing mutations in PHD domains of other proteins. From a comparison of this PHD structure with previously reported RING and LIM (Lin11/Isl-1/Mec-3) structures, we infer sequence determinants that allow discrimination among PHD, RING and LIM motifs.