Synchronized secretion of SBP-EGFP-ColX using the SPI assay. HeLa cells stably expressing SBP-EGFP-ColX and seeded on anti-GFP–coated coverslips (SPI assay) were observed in real time using a spinning disk microscope. A projection of several Z-slices is shown. Time is indicated in minutes:seconds. Biotin was added at frame 2 (00:00). Corresponds to Fig. 1 C.
