HLA-Class II-Invariant Chain Complexes Figure 8. Metabolic labeling reveals that only one DRb chain associated to an Ii homotrimer. The composition of DR-Ii complexes in pulse labeled immunoprecipitates from MelJuSo cells, stably transfected with DRbV5 encoding cDNA. COS cells and MelJuSo cells expressing MHCII subunits and Ii were pulse labeled for 10 min with 35S-methionine. Cells were lysed and immunoprecipitated for Ii (monoclonal antibody Bu45, lane 1), for DRb (S35, lane 2), for DRa (monoclonal antibody TAL-1B5, lane 3), for DRbV5 (monoclonal antibody V5, lanes 4 and 5), for DRab (monoclonal antibody I251SB, lanes 6 and 7) and as a control of reactivity of V5 monoclonal antibody with untransfected MelJuSo cell extract (monoclonal antibody V5, lane 8). Lanes 9 and 10 show molecular weight standards. In lane 11 a DRbV5 immunoprecipitate of MelJuSo cells which were pulse labeled for 10 min with 35S methionine and subsequently cultured for 40 min with medium containing non-radioactive methionine is shown. Immunoprecipitates from lanes 4 and 5 (left panel) were separated on a 20 cm long SDS PAGE gel to separate DRbV5 and DRa chains (right panel). doi:10.1371/journal.pone.0017257.g008 Figure 9. Modeling of MHCII-Ii structure. (left) Molecular model (see Methods section for details) for a pentameric MHCII-Ii oligomer consisting of a trimeric Ii (chains are in blue, red and green cartoons) and one bound MHC-II heterodimer (alpha and beta chains in pink and brown cartoon, respectively). The position of the membrane is indicated (bars representing approximately the lipid-solvent boundary). (right) Top view of the model (from the ER) along the axis of the coiled-coil structure. doi:10.1371/journal.pone.0017257.g009 PLoS ONE | www.plosone.org 9 February 2011 | Volume 6 | Issue 2 | e17257