SIGLEC5 (sialic acid binding Ig-like lectin 5 )

Siglec-5 is a sialic acid-recognizing inhibitory receptor found on certain immune cells types that is undergoing concerted evolution with the gene for Siglec-14 (an activatory receptor) via ongoing gene conversion of exons encoding amino-terminal V-set lg-like domains.  The result is that the two molecules have similar binding properties for sialic acids. However surprisingly, a critical arginine residue required for sialic acid recognition has been mutated in the corresponding molecules from the chimpanzee, gorilla and orangutan. It appears that this may be the ancestral condition, and that the human molecules have restored the arginine residue required for sialic acid recognition. It is unclear what the evolutionary significance of this adjustment is, and further studies are needed. It is interesting to note that a human-specific pathogen called Group B streptococcus can express a protein that ligates Siglec-5 and send a negative inhibitory signal to white blood cell