LIMULUS FACTOR-D, A 43-KDA PROTEIN ISOLATED FROM HORSESHOE-CRAB HEMOCYTES, IS A SERINE-PROTEASE HOMOLOG WITH ANTIMICROBIAL ACTIVITY

A glycoprotein (M(r)=43000) from horseshoe crab hemocytes with antimic robial activity against Gram-negative bacteria was purified, The inter nal peptide sequences coincided exactly with the deduced amino acid se quence of a cDNA clone, designated limulus factor D, which was isolate d by screening a hemocyte cDNA library with an anti-human plasminogen antibody, The open reading frame codes for a precursor of factor D of 394 amino acid residues, including an NH2-terminal signal sequence. Th e COOH-terminal domain of factor D has significant sequence homology w ith the catalytic domain of mammalian serine proteases, in particular with human tissue plasminogen activator (32% identity), except for the substitution of Ser of the active site tried to Gly. Factor D has a u nique NH2-terminal domain with weak sequence homology with part of the mammalian interleukin-6 receptor alpha-chain. Factor D is likely to h ave an important role in host defense mechanisms.