Regulation of murine MHC class II molecule expression. Identification of A beta residues responsible for allele-specific cell surface expression.

Abstract

A panel of mutant class II genes have been constructed using site-directed mutagenesis and DNA-mediated gene transfer. Using this technique, A.beta.k polypeptides have been altered by substituting one or more A.beta.d-specific residues at polymorphic positions in the .beta.1 domain. Transfection of M12.C3 B lymphoma cells with most mutant A.beta.k* genes results in the expression of A.beta.k* A.alpha.d molecules on the cell surface. However, the substitution of a single d allele residue at position 78 or 86 in the A.beta.k polypeptide results in either the complete absence or very low levels, respectively, of cell surface expression of the A.beta.k* A.alpha.d molecule, but does not alter A.beta.k* A.alpha.k expression. The T.86 A.beta.k* A.alpha.d is expressed primarily in an intracellular compartment while the T.78 A.beta.k* A.alpha.d molecule does not appear to be produced. The core-glycosylated T.78 A.beta.k* polypeptide does, however, form a complex intracellulary with the coreglycosylated Ii polypeptide. Substitution of the combination of d allele residues at A.beta.k polymorphic positions (, 12, 13, 14, and 17 results in the absence of A.beta.k* A.alpha.k cell surface expression but does not alter the expression of this mutant A.beta.k* polypeptide with the A.alpha.d polypeptide. These allele-specific expression mutants demonstrate that substitution at certain B1 domain positions may result in the alteration of Ia cell surface expression and that the transport of Ia molecules from the Golgi apparatus to the cell surface may be regulated by signals that are determined by the interaction of polymorphic residues in both the .alpha. and .beta. polypeptides.