The invariant chains of mouse class II antigens: biochemical properties and molecular relationship.

The proteins p40 (Mr = 40 000), p32 (Mr = 32 000), p28 (Mr = 28 000), p20 (Mr = 20 000) and p10 (Mr = 10 000) are described which occur in noncovalent association with the polymorphic alpha, beta heterodimer of class II antigens. They were investigated with respect to their molecular characteristics and their mutual structural relationship. p32, the predominant species of this group corresponds to the invariant chain gamma (Ii). In contrast to the polymorphic subunits alpha and beta, proteins p40, p28, p20 and p10 migrated like gamma in electrophoretically constant positions, when class II molecules of different subregions and different alleles were assessed by two-dimensional gel electrophoresis [1st dimension, isoelectric focusing; 2nd dimension, sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis]. Analogous to gamma, they are therefore designated invariant chains. The low Mr species of this group do not arise from higher Mr forms as preparation artefacts. Short-term pulse-chase analysis and cell-free translation of sucrose gradient-fractionated mRNA in conjunction with specific immunoprecipitation rendered the possibility unlikely that individual components of this set of proteins existed in a precursor-product relationship within the cell. Comparative enzymatic fragmentation on SDS polyacrylamide gels as well as tryptic peptide map comparisons by high performance liquid chromatography revealed a high structural relatedness among all members of this group of invariant proteins.