Cross‐reactivity of the anti‐La monoclonal antibody SW5 with early endosome antigen 2

Summary Coimmunoprecipitation studies with SW5, a frequently used and specific mouse monoclonal antibody (mAb) directed against the human La autoantigen, led to the identification of a functionally unrelated 80 000 MW protein, designated early endosome antigen 2 (EEA2). EEA2 appeared to be directly targeted by mAb SW5. Because an RNA‐binding domain, a structural element of La containing the SW5‐epitope, was not discernable in the primary structure of EEA2, the SW5‐epitope on EEA2 was determined. Coiled‐coil region 3 of EEA2 appeared to contain the epitope recognized by SW5. The SW5 epitope regions of La and EEA2 share a limited sequence homology and probably share a higher degree of structural similarity at the tertiary level. Most likely, the most critical determinants for recognition by SW5 reside in elements adopting alpha‐helical conformations. These data indicate that the application of specific mAbs to purify and characterize (functionally) interacting proteins can be severely obscured by the cross‐reactivity of mAbs with structurally, but not functionally, similar proteins.