STRUCTURE OF A TRUNCATED HUMAN SURFACTANT PROTEIN-D IS LESS EFFECTIVEIN AGGLUTINATING BACTERIA THAN THE NATIVE STRUCTURE AND FAILS TO INHIBIT HEMAGGLUTINATION BY INFLUENZA-A-VIRUS

Surfactant protein D (SP-D) is a lung-specific protein that is synthes ized and secreted by lung epithelial cells and is believed to play an important role in lung host defence. This protein belongs to the C-typ e lectin family, which is characterized by an N-terminal cysteine-rich domain, a collagen-like domain, a neck domain and a carbohydrate reco gnition domain (CRD). To elucidate the biological actions of this anim al lectin against such pathogens as micro-organisms, the biological ac tivities of a recombinant partial SP-D lacking a collagen-like domain were examined. A recombinant human SP-D, consisting of a short collage n region (two repeats of Gly-Xaa-Yaa amino acid sequences), the neck d omain and the CRD, was expressed in Escherichia coli. The recombinant SP-D was purified on a nickel column and then on a maltose-agarose col umn. This protein can form a trimeric structure owing to the neck doma in and exhibits sugar-binding activity and specificity similar to thos e of native human SP-D. The recombinant SP-D caused dose-dependent and calcium-dependent agglutination of E. coli Y1088. The agglutination t itre (the concentration required to achieve a 50% decrease in light tr ansmission by agglutination) of recombinant SP-D was approx. 6-fold th at of native SP-D. As for conglutination, the recombinant trimeric con glutinin required 8-16-fold higher concentrations than the native coun terpart. In haemagglutination inhibition (HI) of influenza A virus, al though native and recombinant conglutinin showed similar levels of HI activity, the recombinant SP-D was unable to inhibit haemagglutination , even at a concentration approx. 120-fold that of the native SP-D. Th e lectin precipitation and lectin blot assays showed that the truncate d SP-D could bind to influenza A virus as well as native SP-D did. The se results indicate that the agglutination activity of trimeric collec tins can be largely retained, and furthermore that the oligomeric stru cture with several hands at opposite sites can enhance agglutination a ctivity. The difference in HI activity against influenza A virus betwe en native and recombinant SP-D suggests that SP-D uses a different mec hanism from that of conglutinin to inhibit viral haemagglutination.