Recombinant human interleukin-12 is the second example of a C-mannosylatedprotein

The beta-chain of human interleukin 12 (IL-12) contains at position 319-322 , the sequence Trp-x-x-Trp, In human RNase 2 this is the recognition moth f or a new recently discovered posttranslational modification, i.e., the C-gl ycosidic attachment of a mannosyl residue to the side chain of tryptophan, Analysis of C-terminal peptides of recombinant IL-12 (rHuIL-12) by mass spe ctrometry and NMR spectroscopy revealed that Trp-319 beta is (partially) C- mannosylated, This finding was extended by in vitro mannosylation experimen ts, using a synthetic peptide derived from the same region of the protein a s an acceptor. Furthermore, human B-lymphoblastoid cells, which secrete IL- 12, were found to contain an enzyme that carries out the C-mannosylation re action. This shows that nonrecombinant IL-12 is potentially C-mannosylated as well. This is only the second report on a C-mannosylated protein. Howeve r, the occurrence of the C-mannosyltransferase activity in a variety of cel ls and tissues, and the presence of the recognition motif in many proteins indicate that more C-mannosylated proteins may be found.