Structure-function analysis of CD14 as a soluble receptor for lipopolysaccharide

CD14 is a glycophosphatidylinositol-linked protein expressed by myeloid cel ls and also circulates as a plasma protein lacking the glycophosphatidylino sitol anchor. Both membrane and soluble CD14 function to enhance activation of cells by lipopolysaccharide (LPS), which we refer to as receptor functi on. We have previously reported the LPS binding and cell activation functio ns of a group of five deletion mutants of CD14 (Viriyakosol, S., and Kirkla nd, T.N. (1995) J. Biol. Chem. 270, 361-368). We have now studied the funct ional impact of these mutations on soluble CD14. We found that some deletio ns that abrogated LPS binding in membrane CD14 have no effect on LPS bindin g in soluble CD14. In fact, some of the soluble CD14 deletion mutants bound LPS with an apparent higher affinity than wild-type CD14. Furthermore, we found that all five deletions essentially ablated soluble CD14 LPS receptor function, whereas only two of the deletions completely destroyed membrane CD14 LPS receptor function. Some of the mutants were able to compete with w ild-type CD14 in soluble CD14-dependent assays of cellular activation. We c oncluded that the soluble and membrane forms of CD14 have different structu ral determinants for LPS receptor function.