Immunologically discrete conformation isomers of I-A locus-equivalent class II molecules detected in Lewis rats

Abstract

Monoclonal antibodies MRC-OX6 and MRC-OX3 were used to define biosynthetically inter-related subsets of I-A-equivalent class II molecules from Lewis rat spleen cells. MRC-OX6 was shown to recognize specifically multiple forms of the class II molecule arising along its maturation pathway from the origin of polypeptide chain synthesis, the rough endoplasmatic reticulum, through the Golgi compartment to the plasma membrane. Three MRC-OX6-reactive polypeptide chain complexes were distinguished: (a) an early complex composed of immature components of the polymorphic alpha,beta heterodimer in noncovalent association with immature proteins of the invariant gamma-chain group p40, p33 (gamma), p28, p20; (b) a biosynthetic intermediate comprising the subunits alpha,beta and gamma all of them being extensively glycosylated and sialylated. The latter constituent is referred to as p36; (c) a cell surface structure consisting of the mature alpha,beta heterodimer devoid of the mature invariant chain p36. A two-dimensional (2D) separation pattern exhibited by an MRC-OX6-specific immunoprecipitate from spleen cells labeled for 4 h represents a superposition of these three MRC-OX6-reactive polypeptide chain complexes. In contrast, MRC-OX3 recognizes exclusively a fully mature alpha,beta heterodimer devoid of protein precursor forms and devoid of any invariant chains. Considering the previous observation that the respective alpha and beta subunits of MRC-OX6 and MRC-OX3-specific molecules comigrate on 2D O'Farrell gels combined with the data of this investigation, it is suggested that the mature MRC-OX6-specific alpha,beta heterodimer and the mature MRC-OX3-specific alpha,beta heterodimer originate from the same biosynthetic intermediate. We propose that before entering the plasma membrane an MRC-OX6-reactive molecule composed of fully glycosylated alpha, beta and p36 releases its post-translationally processed gamma chain (p36), giving rise to two conformation isomers, one alpha,beta heterodimer still being reactive with MRC-OX6 and one alpha,beta heterodimer which has lost the serologic MRC-OX6 and gained the MRC-OX3 specificity. Both alpha, beta heterodimers are expressed on the cell surface as two distinct subsets of I-A-equivalent class II molecules.