title: Surfactant protein D (Sp-D) binds to membrane-proximal domain (D3) of signal regulatory protein α (SIRPα), a site distant from binding domain of CD47, while also binding to analogous region on signal regulatory protein β (SIRPβ).
abstract: Signal regulatory protein α (SIRPα), a highly glycosylated type-1 transmembrane protein, is composed of three immunoglobulin-like extracellular loops as well as a cytoplasmic tail containing three classical tyrosine-based inhibitory motifs. Previous reports indicate that SIRPα binds to humoral pattern recognition molecules in the collectin family, namely surfactant proteins D and A (Sp-D and Sp-A, respectively), which are heavily expressed in the lung and constitute one of the first lines of innate immune defense against pathogens. However, little is known about molecular details of the structural interaction of Sp-D with SIRPs. In the present work, we examined the molecular basis of Sp-D binding to SIRPα using domain-deleted mutant proteins. We report that Sp-D binds to the membrane-proximal Ig domain (D3) of SIRPα in a calcium- and carbohydrate-dependent manner. Mutation of predicted N-glycosylation sites on SIRPα indicates that Sp-D binding is dependent on interactions with specific N-glycosylated residues on the membrane-proximal D3 domain of SIRPα. Given the remarkable sequence similarity of SIRPα to SIRPβ and the lack of known ligands for the latter, we examined Sp-D binding to SIRPβ. Here, we report specific binding of Sp-D to the membrane-proximal D3 domain of SIRPβ. Further studies confirmed that Sp-D binds to SIRPα expressed on human neutrophils and differentiated neutrophil-like cells. Because the other known ligand of SIRPα, CD47, binds to the membrane-distal domain D1, these findings indicate that multiple, distinct, functional ligand binding sites are present on SIRPα that may afford differential regulation of receptor function.
authors.affiliation: Epithelial Pathobiology Research Unit, Department of Pathology and Laboratory Medicine, Emory University School of Medicine, Atlanta, Georgia 30322, USA. bfourni@emory.edu
authors.forename: Bénédicte
authors.initials: B
authors.lastname: Fournier
authors.forename: Rakieb
authors.initials: R
authors.lastname: Andargachew
authors.forename: Alexander Z
authors.initials: AZ
authors.lastname: Robin
authors.forename: Oskar
authors.initials: O
authors.lastname: Laur
authors.forename: Dennis R
authors.initials: DR
authors.lastname: Voelker
authors.forename: Winston Y
authors.initials: WY
authors.lastname: Lee
authors.forename: Dominique
authors.initials: D
authors.lastname: Weber
authors.forename: Charles A
authors.initials: CA
authors.lastname: Parkos
mesh_terms: D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D000818:Animals; D000943:Antigens, Differentiation / Q000235:genetics / Q000378:metabolism*; D051928:CD47 Antigen / Q000235:genetics / Q000378:metabolism*; D016466:CHO Cells; D002118:Calcium / Q000378:metabolism*; D006224:Cricetinae; D003412:Cricetulus; D006031:Glycosylation; D057809:HEK293 Cells; D018922:HL-60 Cells; D006801:Humans; D009504:Neutrophils / Q000378:metabolism*; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D037663:Pulmonary Surfactant-Associated Protein D / Q000235:genetics / Q000378:metabolism*; D011971:Receptors, Immunologic / Q000235:genetics / Q000378:metabolism*; D017384:Sequence Deletion
chemical_list: D000943:Antigens, Differentiation; D051928:CD47 Antigen; C497894:CD47 protein, human; D037663:Pulmonary Surfactant-Associated Protein D; D011971:Receptors, Immunologic; C501920:SIRPA protein, human; D002118:Calcium