Posttranslational modifications of the Ia-associated invariant protein p41 after gene transfer

Biochemical analysis of a rat fibroblast cell clone, transfected with the murine Ia associated invariant chain gene, demonstrates the expression of a family of proteins. This indicates that all members of the invariant protein family are derived from the same gene. The proteins p41, Ii, p27, p25, and p10, synthesized in the transfectant cell line, are identical with the invariant proteins previously shown to associate noncovalently with Ia antigens. These proteins are identified by immunoprecipitation and western blotting with a monoclonal antibody against the N-terminus and with an antiserum against the C-terminal part of the invariant chain. One protein, p41, has recently been shown to contain a thyroglobulin repeat (TgR) element. It was suggested that p41 might use the TgR element as a signal sequence which guides its intracellular transport to endosomes or lysosomes. Here, I demonstrate that p41 binds four N-linked carbohydrates and is heavily sialylated. During transport through trans Golgi compartments p41 binds palmitic acid, presumably at the same cytoplasmic cysteine as previously shown for Ii. A consensus sequence surrounding the palmitylated cysteine of the invariant chains (Ii, p41) and the human transferrin receptor was found. The transferrin receptor is known to follow an endocytic pathway, for which its cytoplasmic domain is essential. It is conceivable that the palmitylated domain of the invariant chains is a guiding structure for the membrane fusion process with transport vesicles. A role of the invariant proteins for antigen processing/presentation is discussed.