Normal Ligand Binding and Signaling by CD47 (Integrin-associated Protein) Requires a Long Range Disulfide Bond between the Extracellular and Membrane-spanning Domains

Abstract

CD47 is a unique member of the Ig superfamily with a single extracellular Ig domain followed by a multiply membrane-spanning (MMS) domain with five transmembrane segments, implicated in both integrin-dependent and -independent signaling cascades. Essentially all functions of CD47 require both the Ig and MMS domains, raising the possibility that interaction between the two domains is required for normal function. Conservation of Cys residues among CD47 homologues suggested the existence of a disulfide bond between the Ig and MMS domains that was confirmed by chemical digestion and mapped to Cys33 and Cys263. Subtle changes in CD47 conformation in the absence of the disulfide were suggested by decreased binding of two anti-Ig domain monoclonal antibodies, decreased SIRPαi binding, and reduced CD47/SIRPα1-mediated cell adhesion. Mutagenesis to prevent formation of this disulfide completely disrupted CD47 signaling independent of effects on ligand binding, as assessed by T cell interleukin-2 secretion and Ca 2+ responses. Loss of the disulfide did not affect membrane raft localization of CD47 or its association with αvβ 3 integrin. Thus, a disulfide bond between the Ig and MMS domains of CD47 is required for normal ligand binding and signal transduction.