(A) Tapasin and B8 were expressed at similar levels in B8+wt mTsn relative to B8+sol mTsn. Samples of lysates from .220, .220-B8, .220-B8+wt mTsn, and .220-B8+sol mTsn were electrophoresed on a 10% acrylamide Tris-glycine gel for tapasin (Tsn) or on a 4→20% acrylamide Tris-glycine gel for B8, the proteins were transferred to a blotting membrane and probed with Ab specific for mouse tapasin (Tsn) or with HC10 to identify the open B8 heavy chain (HC). (B) Soluble mouse tapasin does not associate with B8. Open B8 heavy chains were immunoprecipitated from a lysate of each of the indicated cell types with the HC10 mAb. The immunoprecipitates were electrophoresed on acrylamide Tris-glycine gels, transferred to membranes, and probed with HC10 to identify the immunoprecipitated open B8 heavy chain (HC) or with anti-mouse tapasin antibody (Tsn) to identify the co-immunoprecipitated tapasin. (C) Soluble mouse tapasin does not associate with human TAP. TAP1 was immunoprecipitated from cell lysates with antibody 148.3. After electrophoresis of the immunoprecipitated proteins on a 10% acrylamide Tris-glycine gel, the proteins were blotted and probed with anti-mouse tapasin antibody (Tsn).