BIOSYNTHESIS AND INTRACELLULAR-TRANSPORT OF MHC CLASS-II MOLECULES ASSOCIATED WITH A MUTATED, GLYCOSAMINOGLYCAN-NEGATIVE INVARIANT CHAIN

The MHC Class II molecular complex is composed of polymorphic α and β chains and a non-polymorphic "invariant" chain (Ii) that can be converted to a chondroitin sulfate proteoglycan. To determine whether the proteoglycan form of invariant chain (Ii-CS) had a role in the expression of the Class II complex, we studied the biosynthetic fate of α-β in cells transfected either with normal li cDNA or with a site-directed mutant of Ii (IiAla201) that lacked the site of glycosaminoglycan addition. We had reported [Miller J., Hatch J. A., Simonis S. and Cullen S. E. (1988) Proc. Natl. Acad. Sci. U.S.A. 85, 1359-1363] that the mutant protein associated stably with α and β chains, and that it showed charge heterogeneity suggesting some oligosaccharide processing. In this study we demonstrated that the rate and extent of α-β processing and the rate of α-β cell surface expression is the same in the presence of either normal or mutant Ii. Examination of the mutant Ii protein itself revealed normal transport through different processing compartments, as evidenced by the addition of fatty acid and by maturation of N-linked oligosaccharides. Moreover, though the rate of conversion of IiAla201 into more processed forms was slower in the absence of α-β than in its presence, this was also typlcal of the wild type invariant chain. The ability to alter glycosaminoglycan addition without significantly disturbing other processing events or trafficking should allow a rigorous assessment of the impact of glycosaminoglycan addition on Class II function. © 1990.