Coimmunoprecipitation of apo(a) and β-2 glycoprotein I from COS-1 supernatants. COS-1 cells were transiently transfected with empty vector control (upper panel) or two recombinant apo(a) isoforms [r-apo(a)K11, middle panel; r-apo(a)K26, lower panel]. Following transfection, purified β-2 glycoprotein I was added to COS-1 cellular supernatants and binding of β-2 glycoprotein I to the two different r-apo(a)s was shown using a coimmunoprecipitation assay. Immunoprecipitates were formed by using two different MoAbs (F7, F10) against β-2-glycoprotein I or nonimmune serum. Immunoprecipitation of apo(a) with the apo(a) specific MoAb, 1A2, served as positive control. Immunoprecipitates were resolved by 4% to 12% Tris-Glycin gel electrophoresis and immunodetected by immunoblot analysis using anti-apo(a) MoAb 1A2.