Protein mobilities and P-selectin storage in Weibel-Palade bodies

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 mobile; however, following insertion into immature WPBs larger molecules (VWF, Proregion, tPA) and P-selectin became immobilised, whereas small proteins (ssEGFP, eotaxin-3) became less mobile. WPB maturation led to further decreases in mobility of small proteins and CD63. Acute alkalinisation of mature WPBs selectively increased the mobilities of small soluble proteins without affecting larger molecules and the membrane proteins. Disruption of the Proregion-VWF paracrystalline core by prolonged incubation with NH₄Cl rendered P-selectin mobile while VWF remained immobile. FRAP of P-selectin mutants revealed that immobilisation most probably involves steric entrapment of the P-selectin extracellular domain by the Proregion-VWF paracrystal. Significantly, immobilisation contributed to the enrichment of P-selectin in WPBs; a mutation of P-selectin preventing immobilisation led to a failure of enrichment. Together these data shed new light on the transitions that occur for soluble and membrane proteins following their entry and storage into post-Golgi-regulated secretory organelles.