Mass Photometry Experimental Protocol

Mass photometry experiments were performed on a Refeyn OneMP mass photometer (Refeyn) similar to described before^{51 (link)}. Microscope coverslips (24 mm × 50 mm; Marienfeld) were prepared by sequential cleaning in sonication baths of isopropanol and then MilliQ water (2x), followed by placement of a CultureWell gasket (Grace Biolabs). About 15 µL of PBS was placed in a well for focusing, after which about 3 µL of diluted sample was mixed in. Measurement concentrations were typically around 5–20 nM. Measurements were recorded using medium field-of-view settings for 120 s. An in-house calibration mix consisting of IgG4Δhinge-L368A (73 kDa), IgG1-Campath (149 kDa), apoferritin (483 kDa), and GroEL (800 kDa) was used. Recordings were processed in DiscoverMP (Refeyn), from which also the relative abundances of protein complexes were obtained. Further data analysis and plotting were performed in Jupyter Notebook using an in-house Python library.