FORMATION OF THE ALPHA(1)-MICROGLOBULIN CHROMOPHORE IN MAMMALIAN AND INSECT CELLS - A NOVEL POSTTRANSLATIONAL MECHANISM

alpha(1)-Microglobulin is an immunosuppressive plasma protein synthesi zed by the liver. The isolated protein is yellow-brown, but the hypoth etical chromophore has not yet been identified. In this work, it is sh own that a human liver cell line, HepG2, grown in a completely synthet ic and serum-free medium, secretes alpha(1)-microglobulin which is als o yellow-brown, suggesting a de novo synthesis of the chromophore by t he cells. alpha(1)-Microglobulin isolated from the culture medium of i nsect cells transfected with the gene for rat alpha(1)-microglobulin i s also yellow-brown, suggesting that the gene carries information abou t the chromophore. Reduction and alkylation or removal of N- or O-link ed carbohydrates by glycosidase treatment did not reduce the colour in tensity of the protein. An internal dodecapeptide (amino acid position s 70-81 in human alpha(1)-microglobulin) was also yellow-brown. The la tter results indicate that the chromophore is linked to the polypeptid e. In conclusion, the results suggest that the alpha(1)-microglobulin gene carries information activating a post-translational protein modif ication mechanism which is present in mammalian and insect cells.