title: Siglec-5 (CD170) can mediate inhibitory signaling in the absence of immunoreceptor tyrosine-based inhibitory motif phosphorylation.
abstract: Siglec-5 (CD170) is a member of the recently described human CD33-related siglec subgroup of sialic acid binding Ig-like lectins and is expressed on myeloid cells of the hemopoietic system. Similar to other CD33-related siglecs, Siglec-5 contains two tyrosine-based motifs in its cytoplasmic tail implicated in signaling functions. To investigate the role of these motifs in Siglec-5-dependent signaling, we used transfected rat basophil leukemia cells as a model system. Tyrosine phosphorylation of Siglec-5 led to recruitment of the tyrosine phosphatases SHP-1 and SHP-2, as seen in both pull-down assays and microscopy. Siglec-5 could efficiently inhibit FcepsilonRI-mediated calcium fluxing and serotonin release after co-cross-linking. Surprisingly, a double tyrosine to alanine mutant of Siglec-5 could still mediate strong inhibition of serotonin release in the absence of detectable tyrosine phosphorylation, whereas a double tyrosine to phenylalanine mutant lost all inhibitory activity. In comparison, suppression of Siglec-5-dependent adhesion to red blood cells was reversed by either tyrosine to alanine or tyrosine to phenylalanine mutations of the membrane proximal tyrosine-based motif. Using an in vitro phosphatase assay with synthetic and recombinant forms of the cytoplasmic tail, it was shown that a double alanine mutant of Siglec-5 had weak, but significant SHP-1 activating properties similar to those of wild type, non-phosphorylated cytoplasmic tail, whereas a double phenylalanine mutant was inactive. These findings establish that Siglec-5 can be classified as an inhibitory receptor with the potential to mediate SHP-1 and/or SHP-2-dependent signaling in the absence of tyrosine phosphorylation.
authors.affiliation: Division of Cell Biology and Immunology, The Wellcome Trust Biocentre, School of Life Sciences, University of Dundee, UK.
authors.forename: Tony
authors.initials: T
authors.lastname: Avril
authors.forename: Sylvie D
authors.initials: SD
authors.lastname: Freeman
authors.forename: Helen
authors.initials: H
authors.lastname: Attrill
authors.forename: Rosemary G
authors.initials: RG
authors.lastname: Clarke
authors.forename: Paul R
authors.initials: PR
authors.lastname: Crocker
mesh_terms: D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D019943:Amino Acid Substitution; D000818:Animals; D015703:Antigens, CD / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D015214:Antigens, Differentiation, Myelomonocytic / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D001483:Base Sequence; D002448:Cell Adhesion / Q000502:physiology; D002460:Cell Line; D004247:DNA / Q000235:genetics; D006801:Humans; D066298:In Vitro Techniques; D047908:Intracellular Signaling Peptides and Proteins; D037102:Lectins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010766:Phosphorylation; D054592:Protein Tyrosine Phosphatase, Non-Receptor Type 11; D053824:Protein Tyrosine Phosphatase, Non-Receptor Type 6; D017027:Protein Tyrosine Phosphatases / Q000235:genetics / Q000378:metabolism; D051381:Rats; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction; D014162:Transfection; D014443:Tyrosine / Q000737:chemistry
chemical_list: D015703:Antigens, CD; D015214:Antigens, Differentiation, Myelomonocytic; D047908:Intracellular Signaling Peptides and Proteins; D037102:Lectins; D011993:Recombinant Fusion Proteins; C114531:SIGLEC5 protein, human; D014443:Tyrosine; D004247:DNA; C516724:PTPN11 protein, human; C509249:PTPN6 protein, human; D054592:Protein Tyrosine Phosphatase, Non-Receptor Type 11; D053824:Protein Tyrosine Phosphatase, Non-Receptor Type 6; D017027:Protein Tyrosine Phosphatases; C516727:Ptpn11 protein, rat; C509250:Ptpn6 protein, rat