The Structural Biology Study Of Human Innate Immune Related Protein SPLUNC1

BPI domain containing superfamily is composed of some lipid binding and transferring proteins. Based on their species and sizes, the superfamily can be divided into to two subfamilies:BPI like family and Takeout like family. PLUNC proteins are newly discovered which are specially expressed in the respiratory system of animals. Gene location and sequence alignment results show that PLUNC proteins may have one or two BPI domains. As the most important one, human SPLUNC1has one BPI domain and it plays a great role in the innate immune system. SPLUNC1can bind the LPS monomer, has the antibacterial activity and is a surfactant protein. Because of the low solubility of SPLUNC1, we fused SPLUNC1to MBP to increase its solubility. Then we got one crystal of the fusion protein after initial trials and optimization. Using singal wavelength annormalous diffraction method, we solved the2.5A structure of Se-SPLUNC3A.SPLUNC1has a BPI barrel which is composed of helices and a beta sheet.SPLUNC1also has a helix α4on its C terminal to close the barrel.Between the β2and β3, there is a conserved hairpin structure which can anchor the C terminal to the area of al and02by forming H bonds between hairpin’s site chains and the main chain of the C terminal region. Two hydrophobic cavities are in the SPLUNC1and SPLUNC1shoud use the one near a4to bind LPS. SPLUNC3A has been truncated on the N terminal by24amino acids and it has neither LPS binding nor antibacterial activity. Based on the similar structure comparison, we proposed that SPLUNC1may expose the hydrophobic cavity to bind LPS by the conformational change of N terminal residues.We can not find the large hydrophobic region on the surface of SPLUNC1to explain its low solubility. More experiments need to be done to explain the antibacterial activity of SPLUNC1.