N-linked glycosylation at Asn3 and the positively charged residues within the amino-terminal domain of the c1 inhibitor are required for interaction of the C1 Inhibitor with Salmonella enterica serovar typhimurium lipopolysaccharide and lipid A.

The C1 inhibitor (C1INH), a plasma complement regulatory protein, prevents endotoxin shock, at least partially via the direct interaction of its amino-terminal heavily glycosylated nonserpin region with gram-negative bacterial lipopolysaccharide (LPS). To further characterize the potential LPS-binding site(s) within the amino-terminal domain, mutations were introduced into C1INH at the three N-linked glycosylation sites and at the four positively charged amino acid residues. A mutant in which Asn(3) was replaced with Ala was markedly less effective in its binding to LPS, while substitution of Asn(47) or Asn(59) had little effect on binding. The mutation of C1INH at all four positively charged amino acid residues (Arg(18), Lys(22), Lys(30), and Lys(55)) resulted in near-complete failure to interact with LPS. The C1INH mutants that did not bind to LPS also did not suppress LPS binding or LPS-induced up-regulation of tumor necrosis factor alpha mRNA expression in RAW 264.7 macrophages. In addition, the binding of C1INH mutants to diphosphoryl lipid A was decreased in comparison with that of recombinant wild-type C1INH. Therefore, the interaction of C1INH with gram-negative bacterial LPS is dependent both on the N-linked carbohydrate at Asn(3) and on the positively charged residues within the amino-terminal domain.