TRANSPORT AND EXPRESSION IN HUMAN MELANOMAS OF A TRANSFERRIN-LIKE GLYCOSYLPHOSPHATIDYLINOSITOL-ANCHORED PROTEIN

Melanotransferrin, also called p97, is a cell surface glycoprotein whi ch was first described as a marker antigen, for human melanoma cells. Although p97 has a striking structural similarity to human serum trans ferrin and lactoferrin, its function has not yet between determined. O ne feature that distinguishes p97 from the other members of the transf errin family is the presence of a stretch of 24 hydrophobic amino acid s at the C terminus, previously assumed to form a proteinacious transm embrane domain. In this study, sensitivity to bacterial phosphatidylin ositol-specific phospholipase C, biosynthetic labeling with [H-3]ethan olamine, and partitioning in Triton X-114 are used to establish that p 97 is expressed at the cell surface as a glycosylphosphatidylinositol- anchored protein. In addition, to gain insight into the intracellular transport of p97, biosynthetic transport studies were performed on a m elanoma cell line. These studies resulted in the identification of an additional form of p97 which is found in the medium and which likely d oes not originate from an alternatively spliced form of the p97 mRNA. These findings, together with our recent observation of the co-localiz ation of p97 and the transferrin receptor in brain capillary endotheli um (W. A. Jefferies, M. R. Food, R. Gabathuler, S. Rothenberger, T. Ya mada, and P. L. McGeer, manuscript submitted) raise important question s about the function of the two forms of p97 detected and the possible involvement of this protein in a cellular iron uptake mechanism that is independent from the transferrin/transferrin receptor system.