The rat OX41 antigen is a cell surface protein containing three immunoglobulin superfamily domains and intracellular immunoreceptor tyrosine‐based inhibitory motifs (ITIM). It is a homologue of the human signal‐regulatory protein (SIRP) also known as SHPS‐1, BIT or MFR. Cell activation‐induced phosphorylation of the intracellular ITIM motifs induces association with the tyrosine phosphatases SHP‐1 and SHP‐2. To identify the physiological OX41 ligand, recombinant OX41‐CD4d3+4 fusion protein was coupled to fluorescent beads to produce a multivalent cell binding reagent. The OX41‐CD4d3+4 beads bound to thymocytes and concanavalin A‐stimulated splenocytes. This interaction was blocked by the monoclonal antibody (mAb) OX101. Affinity chromatography with OX101 mAb and peptide sequencing revealed the rat SIRP ligand to be CD47 (integrin‐associated protein). A direct interaction between human SIRP and human CD47 was demonstrated using purified recombinant proteins and surface plasmon resonance ruling out the involvement of other proteins known to be associated with CD47. The affinity of the SIRP/CD47 interaction was Kd ≈ 8 μM at 37°C with a koff ≥2.1 s–1. The membrane‐distal SIRP V‐like domain was sufficient for binding to CD47.

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We show that Xenopus oocytes and embryos contain GATA-2, stored in nuclei as a non-chromatin-bound complex. Its binding site specificity is different from that of GATA-1, having a much higher affinity for the motif with a core GATC sequence. This binding site preference was markedly reduced by either release of the factor with deoxycholate or purification on a DNA affinity column, suggesting a role for a cofactor(s). The identity of the maternal GATA factor was established as GATA-2 in two ways: (1) binding to an oligonucleotide probe was abolished by inclusion of either of two GATA-2 monoclonal antibodies, and (2) a protein of correct molecular weight for GATA-2 was detected by immunoblotting with a polyclonal antibody raised against a Xenopus GATA-2-specific peptide. Although predominantly complexed, some of the oocyte GATA-2 is functional as a transcription factor because the transcriptional activity of the chicken betaH-globin promoter injected into oocytes was reduced by mutation of either of two GATA binding sites. This effect was more pronounced when the stronger of the two sites was mutated. Butyrate treatment of oocytes stimulated cap-site initiation by up to 17-fold with both normal promoter and GATA site mutant constructs, showing that the mechanism of butyrate stimulation is not via GATA-2. The possible significance of regulating the availability of maternal GATA-2 during early development is discussed.