Mutations in specific I-A(k) alpha(2) and beta(2) domain residues affect surface expression

A previous investigation demonstrated that several mutations in class II di mer-of-dimers contact residues interfere with antigen presentation by trans fectants but not with plasma membrane expression of the mutant class II. In the present study we examined other class II mutations in this region that did inhibit plasma membrane expression of mutant class II molecules. Molec ules containing both mutations H alpha 181D in the alpha(2) domain and E be ta 170K in the beta(2) domain exhibited low plasma membrane expression, but molecules with only one of these mutations were expressed normally. The mu tant class II molecules were transported to organelles that were accessible to a fluid-phase protein, hen egg lysozyme (HEL), Culture of transfectants with lysozyme enhanced the amount of class II compact dimer (alpha beta pl us peptide; CD), and this was especially marked for the class II mutant H a lpha 181D/E beta 170K and for other molecules possessing both mutations. Fo rmation of class II CD was not paralleled by an increase in class II surfac e expression. Thus the joint mutation of H alpha 181 and E beta 170 has two effects. in the absence of high concentrations of exogenous peptide, it pr events efficient CD formation, possibly by affecting invariant chain (li) p roteolysis and/or the stability of the class II after Ii/CLIP is removed. A t high peptide concentrations supplied by exogenous HEL, the mutations allo w CD formation, but not expression of class II on the plasma membrane. Mole cular modeling of the possible interaction of class II and Ii suggests that the mutant amino acids H alpha 181D and E beta 170K, besides affecting the overall stability of class II, might also interact with ii via two loops i n class Il's alpha(2) and beta(2) domains respectively.