Cloning and molecular characterization of two splice variants of a new putative member of the Siglec-3-like subgroup of Siglecs

The sialic acid binding immunoglobulin-like lectin (Siglec) family is a rec ently described member of the immunoglobulin superfamily. Within this Sigle c family there exists a subgroup of molecules which bear a very high degree of homology with the molecule Siglec-3 (CD33), and has thus been designate d the Siglec-3-like subgroup of Siglecs. The members of this subgroup have been localized to chromosome 19q13.4, through both in situ hybridization an d precise genomic mapping at the nucleotide level. Through the positional c loning approach we have identified and characterized a Siglec-like gene (SL G), a putative novel member of the Siglec-3-like subgroup of Siglecs. We ha ve characterized the complete genomic structure of SLC:, as well as two alt ernative splice variants, and determined its chromosomal localization. The short isoform, SLG-S, consists of seven exons, with six intervening introns , while the longer isoform, SLG-L, consists of eight exons and seven interv ening introns. The SLG gene is localized 32.9 kb downstream of Siglec-8 on chromosome 19q13.4. The putative SLG-S and SLG-L proteins, of 477 and 595 a mino acid residues, respectively, show extensive homology to many members o f the Siglec-3-like subgroup. This high degree of homology is conserved in the extracellular Ig-like domains, as well as in the cytoplasmic tyrosine-b ased motifs. Interestingly, the SLG-L protein contains two N-terminal V-set Ig-like domains, as opposed to SLG-S and other Siglec-3-like subgroup memb ers which contain only one such domain. Through RT-PCR we have examined the expression profile of both SLG; splice variants in a panel of human tissue s and have found that SLG-S is highly expressed in spleen, small intestine and adrenal gland, while SLG-L exhibits high levels of expression in spleen , small intestine, and bone marrow. This gene is quite likely the latest no vel member of the CD33-like subgroup of Siglecs, and given its high degree of homology, it may also serve a regulatory role in the proliferation and s urvival of a particular hematopoietic stem cell lineage, as has been found for CD33 and Siglec7. (C) 2001 Academic Press.