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Biosynthesis and intracellular transport of MHC class II molecules associated with a mutated,
S Simonis1, J Miller, S E Cullen
1Department of Microbiology and Immunology, Washington University School of Medicine, St. Louis, MO 63110.
The MHC Class II molecular complex is composed of polymorphic alpha and beta 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 alpha-beta in cells transfected either with normal Ii 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 alpha and beta chains, and that it showed charge heterogeneity suggesting some oligosaccharide processing. In this study we demonstrated that the rate and extent of alpha-beta processing and the rate of alpha-beta 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 alpha-beta than in its presence, this was also typical 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.
The MHC Class II molecular complex is composed of polymorphic alpha and beta 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 alpha-beta in cells transfected either with normal Ii 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 alpha and beta chains, and that it showed charge heterogeneity suggesting some oligosaccharide processing. In this study we demonstrated that the rate and extent of alpha-beta processing and the rate of alpha-beta 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 alpha-beta than in its presence, this was also typical 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.