Identification of p70 and p80 Associations With Class II MHC Molecules and I i

Two proteins, p70 and p80, were found in chemically crosslinked complexes with class II MHC molecules and I i after 3–12 hr labelings with [ 35 S]methionine. Two‐dimensional, nonreduced/reduced SDS gel electrophoresis of immunoprecipitated complexes revealed 1) endogenous disulfide linkages between I i ‐I i and I i ‐p70 and 2) chemically crosslinked, nearest neighbors of α‐β, α‐I i , I i ‐p70, and α‐p80. Although such nearest neighbors within multimeric complexes were identified as dimers in nonreduced/reduced 2D gels, stoichiometries could not be determined in the high molecular weight complex(es), which included α, β, I i , p70, and p80, and were not separated in the first dimension. p80 was not the chondroitin‐sulfate form of I i (I i ‐CS) because it was not electrophoretically heterogeneous and was not sensitive to chondroitinase ABC. p70 was not hsp72/74 detected with C92 or N27 mAbs, and p80 was not BiP detected with its respective mAb. While only these two proteins associated prominently with class II MHC antigens and I i late after synthesis, their functions are unknown.