Using fluorescence recovery after photobleaching (FRAP) we measured the mobilities of EGFP-tagged soluble secretory proteins in the endoplasmic reticulum (ER) and in individual Weibel-Palade bodies (WPBs) at early (immature) and late (mature) stages in their biogenesis. Membrane proteins (P-selectin, CD63, Rab27a) were also studied in individual WPBs. In the ER, soluble secretory proteins were ...
