DELINEATION OF THE SUBUNIT COMPOSITION OF HUMAN PROTEASOMES USING ANTISERA AGAINST THE MAJOR HISTOCOMPATIBILITY COMPLEX-ENCODED LMP2 AND LMP7 SUBUNITS

The products of the Lmp2 and Lmp7 genes located in the major histocomp atibility complex (MHC) class II region are postulated to form part of the proteasome complex. This large, multisubunit complex forms the ma jor, nonlysosomal proteolytic machinery for the degradation of endogen ous proteins and has been suggested to play a role in the processing o f antigens presented by MHC class I molecules. The role of the MHC-enc oded subunits within the proteasome has however remained enigmatic. To study this role, we have raised antibodies to recombinant LMP2 and LM P7 proteins. Under denaturing conditions, the anti-LMP7 serum recogniz es one subunit of proteasome, whereas the anti-LMP2 serum recognizes t wo subunits, which may represent different forms of LMP2. The specific ity of these sera has been ascertained by a lack of reactivity in T2 c ells, which lack both genes. Furthermore under native conditions the a nti-LMP2 serum immunoprecipitates a complex that is similar to proteas ome but lacks several subunits, including LMP7. Preclearing experiment s using this serum and a monoclonal antibody (2-17) specific for the n on-MHC-encoded C2 proteasome subunit demonstrate that the complexes re cognized by these two sera are distinct and that four subunits are uni que to the complex precipitated by the anti-LMP2 serum. Interestingly, the different forms of LMP2 are segregated between these complexes. T he relationship of the two complexes is discussed.