A new siglec family member, siglec‐10, is expressed in cells of the immune system and has signaling properties similar to CD33

The siglecs ( s ialic acid‐binding Ig ‐like lec tins) are a distinct subset of the Ig superfamily with adhesion‐molecule‐like structure. We describe here a novel member of the siglec protein family that shares a similar structure including five Ig‐like domains, a transmembrane domain, and a cytoplasmic tail containing two ITIM‐signaling motifs. Siglec‐10 was identified through database mining of an asthmatic eosinophil EST library. Using the Stanford G3 radiation hybrid panel we were able to localize the genomic sequence of siglec‐10 within the cluster of genes on chromosome 19q13.3‐4 that encode other siglec family members. We have demonstrated that siglec‐10 is an immune system‐restricted membrane‐bound protein that is highly expressed in peripheral blood leukocytes as demonstrated by Northern, RT‐PCR and flow cytometry. Binding assays determined that the extracellular domain of siglec‐10 was capable of binding to peripheral blood leukocytes. The cytoplasmic tail of siglec‐10 contains four tyrosines, two of which are embedded in ITIM‐signaling motifs (Y597 and Y667) and are likely involved in intracellular signaling. The ability of tyrosine kinases to phosphorylate the cytoplasmic tyrosines was evaluated by kinase assay using wild‐type siglec‐10 cytoplasmic domain and Y→F mutants. The majority of the phosphorylation could be attributed to Y597 andY667. Further experiments with cell extracts suggest that SHP‐1 interacts with Y667 and SHP‐2 interacts with Y667 in addition to another tyrosine. This is very similar to CD33, which also binds the phosphatases SHP‐1 and SHP‐2, therefore siglec‐10, as CD33, may be characterized as an inhibitory receptor.