BINDING OF LIPOPOLYSACCHARIDE (LPS) TO AN 80-KILODALTON MEMBRANE-PROTEIN OF HUMAN-CELLS IS MEDIATED BY SOLUBLE CD14 AND LPS-BINDING PROTEIN

Activation of cells by bacterial lipopolysaccharide (LPS) plays a key role in the pathogenesis of gramnegative septic shock. The 55-kDa glyc oprotein CD14 is known to bind LPS and initiate cell activation. Howev er, there must be additional LPS receptors because CD14 is linked by a glycosylphosphatidyl inositol anchor to the cell membrane and therefo re unable to perform transmembrane signalling. Searching for potential LPS receptors, we investigated the binding of LPS to membrane protein s of the human monocytic cell line Mono-Mac-6. Membrane proteins were electrophoretically separated under reducing conditions, transferred t o nitrocellulose, and exposed to LPS, which was visualized with anti-L PS antibody. Smooth- and rough-type LPS, as well as free lipid A, boun d to a variety of proteins in the absence of serum. However, in the pr esence of serum, additional or preferential binding to a protein of ap proximately 80-kDa was observed. Experiments with differently acylated lipid A structures showed that the synthetic tetraacyl compound 406 w as still able to bind, whereas no binding was detected with the bisacy l compound 606. The 80-kDa membrane protein was also detected on human peripheral blood monocytes and endothelial cells. The serum factors m ediating the binding of lipid A to the 80-kDa membrane protein were id entified as soluble CD14 and LPS-binding protein. From these results, we conclude that this 80-kDa protein is a candidate for the hypothetic al molecule for LPS and/or LPS-CD14 recognition and signal transductio n.