Historical Background, Structure, and Its Ligands
Signal regulatory protein alpha (SIRPα) was initially cloned as a substrate for Src homology region 2 (SH2) domain-containing phosphatase-1 (SHP-1) (Ptpn6) and SHP-2 (Ptpn11). SHP-1 and SHP-2 are cytoplasmic-type protein tyrosine phosphatases and SIRPα was initially termed SHPS-1 (SHP substrate-1) (reviewed in Matozaki et al. 2009; Barclay and van den Berg 2014). SIRPα was also named as brain immunoglobulin (Ig)-like molecule with tyrosine-based activation motifs (BIT), which was a highly phosphorylated glycoprotein in the brain, as well as macrophage fusion receptor (MFR) and MyD-1. SIRPα belongs to SIRP family members, and other two SIRP family members, namely SIRPβ1 and SIRPγ, have a similar structure of SIRPα in their extracellular regions but have different cytoplasmic regions from SIRPα (Matozaki et al. 2009; Barclay and van den Berg 2014).
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