Title : Identification of
the glycosaminoglycan-attachment site of mouse invariant-chain proteoglycan core protein by site-directed mutagenesis
Abstract :
- The invariant chain (Ii) , a nonpolymorphic glycoprotein that associates with the immunoregulatory Ia proteins encoded by the major histocompatibility complex, has a proteoglycan form ( Ii-CS) that bears a chondroitin sulfate glycosaminoglycan
- In this proteoglycan form , Ii may remain associated with Ia at the cell surface
- Inhibitors that prevent the addition of glycosaminoglycan to Ii have been found to depress antigen-presenting function
- Ii does not have multiple candidate glycosaminoglycan-attachment sites , and we used site-directed mutagenesis to replace a candidate serine glycosaminoglycan-acceptor site serine glycosaminoglycan-acceptor site with alanine at position 201 in the murine Ii protein
- Transfection of the normal or altered gene into Ii-negative COS-7 cells showed that equivalent amounts of core Ii protein and its acidic, terminally glycosylated forms were synthesized, but the Ala-201 mutant Ii did not give rise to Ii-CS
- The mutant protein had apparently normal transport through the Golgi compartment and associated stably with Ia molecules
- Thus, this mutation directly identifies the site of glycosaminoglycan addition and shows that it can be eliminated without adversely affecting the overall biosynthesis of Ii