Co-IPs for proteomic analysis (mass spectrometry) were carried out by incubation of 1 mg of protein lysates with protein G-coated magnetic beads coupled with either the monoclonal antibody P4A4 anti-endoglin (Developmental Studies Hybridoma Bank, The University of Iowa, US) or an isotype-matched (IgG2b) control antibody (Immunostep, Salamanca, Spain). An additional control with protein G magnetic beads in the absence of antibodies was also included. After extensive washing with PBS, immunoprecipitates were then subjected to mass spectrometry analysis.