ADP-ribosyltransferase activity was shown to be present on the surface of human monocytes. Incubating the cells in the presence of BSA leads to an increase in enzyme activity. The acceptor amino acid mainly responsible for the ADP-ribose bond was identified as a cysteine residue. An increase in ADP-ribosyltransferase activity was observed when cells were treated for 16h with bacterial lipopolysaccharide (LPS). Possible candidates for catalysing the reaction are mono-ADP-ribosyltransferases (ARTs). When measuring expression of the mRNA of ART1, 3, 4 and 5, only ART3 mRNA was detected in unstimulated monocytes. Upon stimulation for 16h with LPS, lipoteichoic acid or peptidoglycan, ART4 mRNA was found to be expressed. No ART4 signal appeared after a 4h exposure of the cells to LPS. Cell-surface proteins were labelled when incubating monocytes with [32P]NAD+. Their molecular masses were 29, 33, 43, 45, 60 and 82kDa. In response to LPS an additional protein of 31kDa was found to be labelled. The bound label was resistant to treatment with NH2OH but sensitive to HgCl2, characteristic of a cysteine-linked ADP-ribosylation.
Abbreviations used: ART, mono-ADP-ribosyltransferase; GPI, glycosylphosphatidylinositol; LPS, lipopolysaccharide; RT, reverse transcriptase; mAb, monoclonal antibody; GAPDH, glyceraldehyde-3-phosphate dehydrogenase; GPI, glycerophosphatidylinositol.