Control and βIRKO beta cell lines were cultured in DMEM as described [29 (link), 30 (link)] and used between passages 11 to 29. Absence of mycoplasma contamination was confirmed every 6 months. Briefly, for acute stimulation, cells cultured in DMEM with 3 mmol/l glucose and/or 10 µmol/l nifedipine (Sigma-Aldrich, St Louis, MO, USA) for 16h were subsequently stimulated with GLP-1 (10 nmol/l) and/or human insulin (5 or 100 nmol/l; Sigma-Aldrich) for 15 min, and protein samples were extracted immediately. For chronic stimulation, cells cultured in DMEM containing 3 mmol/ l glucose and/or 10 nmol/l exendin-4 (Sigma-Aldrich) and/or 200 nmol/l OSI-906 (Selleck, Houston, TX, USA) for 24 h. For protein-stability analysis, control cells were treated with 100 µg/ml cycloheximide (CHX; Sigma-Aldrich). See electronic supplementary material (ESM) Methods for further details. In general, we performed at least three independent experiments for statistical analyses.