The myeloid differentiation antigen CD14 is N- and O-glycosylated - Contribution of N-linked glycosylation to different soluble CD14 isoforms

The myeloid differentiation antigen CD14 acts as the major receptor for bacterial lipopolysaccharide (LPS). A soluble form of the protein (sCD14) is present in human serum which functions as a soluble LPS receptor. We have compared the isoform patterns of soluble CD14 derived from human serum and of the recombinant proteins produced by CHO cells transfected with either the wild-type CD14 gene or with a cDNA coding for a truncated protein which lacks the C-terminal 21 amino acids [sCD14-(1-335)-peptide]. Using SDS/PAGE, two dominant isoforms (53 and 50 kDa) and two minor forms (46 and 43 kDa) can be detected in serum as well as in the supernatants of both transfectants. sCD14 is a glycoprotein which carries N- and O-linked carbohydrates. The different isoforms of sCD14-(1-335)-peptide are due to differences in the content of N-linked sugars. However after the removal of N- and O-linked carbohydrates from serum- and CHO-derived wild-type proteins, two isoforms are still present. These results indicate that N-linked glycosylation contributes to but does not fully explain the different forms of soluble CD14. We further examined whether the mutation of individual N-linked glycosylation sites influences the expression of membrane-bound and soluble CD14 forms and the ability of the membrane-bound molecule to bind LPS. As with the wild-type proteins, the different isoforms of the soluble mutants are partially due to differences in N-linked glycosylation. A truncated mutant which lacks the two N-terminal glycosylation sites {[Asp18, Asp132]CD14-(1-335)peptide} does not give rise to multiple forms on SDS gels. Like CD14-(1-335)-peptide, this mutant is not expressed on the cell surface suggesting that a smaller isoform present in the wild-type preparations results from proteolytic cleavage of the membrane-bound molecule. N-linked carbohydrates do not seem to be important for the binding of LPS to membrane-bound CD14.