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Under shear stress, leukocytes use P-selectin glycoprotein ligand-1 (PSGL-1) to tether to and roll on P-selectin expressed on activated platelets or endothelial cells. P-selectin has an NH2-terminal lectin domain, an epidermal growth factor (EGF)-like motif, nine consensus repeats (CRs), a transmembrane domain, and a cytoplasmic tail. To determine whether the CRs are required for P-selectin to bind PSGL-1, we expressed a soluble protein (Lec-EGF) that contained only the lectin and EGF domains, plus a short C-terminal epitope tag. Electron microscopy and hydrodynamic analysis confirmed that Lec-EGF was monomeric, as previously shown for soluble P-selectin (sPS) that contained the lectin and EGF domains plus all nine CRs. Fluid-phase Lec-EGF or sPS inhibited binding of oligomeric125I-labeled membrane-derived P-selectin (mPS) to PSGL-1 on neutrophils and binding of 125I-PSGL-1 to immobilized mPS. The IC50 for inhibiting binding of mPS to neutrophils was fivefold greater for Lec-EGF than for sPS, whereas the IC50 for inhibiting binding of mPS to purified PSGL-1 was indistinguishable for Lec-EGF and sPS. Under static or shear conditions, neutrophils used PSGL-1 to tether to or roll on Lec-EGF that was captured by an immobilized monoclonal antibody to the C-terminal epitope. These data show that P-selectin requires only the lectin and EGF domains to bind to PSGL-1.

AML1/ETO is the chimeric fusion protein resulting from the t(8;21) found in AML of the M2 subtype. It contains the N-terminal 177 amino acids of RUNX1 and virtually all (575aa) of ETO. The RUNX1 component includes the Runt domain, which mediates both DNA binding and heterodimerization with CBFβ, but lacks the more C-terminal sequences required for transactivation. AML1/ETO occupies RUNX target genes in vivo and is associated with a repressive chromatin structure characterized by reduced levels of acetylated histone H3. AML1/ETO is thought to repress transcription by recruiting a SMRT (N-CoR)/Sin3A/HDAC complex to chromatin via sequences in ETO. ETO is the human homologue of the Drosophila Nervy protein and shares 4 regions of homology with Nervy called Nervy Homology Regions (NHR) 1–4. Deletion studies have shown that three of the AML1/ETO domains essential for its repressive function are the Runt domain, NHR2, and NHR4. The NHR2 domain is a hydrophobic heptad repeat that mediates oligomerization of AML1/ETO, interaction with ETO family members, and also with mSin3A and HDACs. We recently solved an x-ray structure of the NHR2 domain and found it to be an alpha-helical tetramer. Based on this structure we have introduced amino acid substitutions into the NHR2 domain that disrupt tetramer formation but not AML1/ETO stability. These mutations impair the ability of AML1/ETO to inhibit the differentiation of GR-1+/Mac-1+ cells following retroviral transduction into primary mouse bone marrow cells, and also inhibit the serial replating ability of AML1/ETO expressing bone marrow cells in vitro. We additionally show that mutations reported by Amann et al. (Mol Cell Biol. 21, 6470, 2001) to disrupt mSin3A binding to NHR2 do not affect the biological activity of AML1/ETO in vitro. We also introduced mutations in the Runt domain of AML1/ETO that disrupt CBFβ binding by defined amounts (40-fold, 200-fold, 500-fold), and demonstrated that CBFβ binding by AML1/ETO is essential for its dominant negative activity. The latter results suggest that small molecules designed to selectively impair heterodimerization of AML1/ETO with CBFβ could potentially block AML1/ETO’s dominant negative activity.