Intracellular localization and regulated secretion of wild-type VWF propeptide (VWFpp) and Δ437--442-VWFpp. (A) AtT-20 cells were transfected with wild-type VWFpp (1) or Δ437--442-VWF (2, 3). Transfected cells were stained for VWFpp and adrenocorticotrophic hormone (ACTH) (1, 2), or VWFpp and GRP78 (3). Intracellular localization of proteins was examined by confocal microscopy. The merged image of the two stains is shown in the last column, with colocalization shown in yellow. Δ437--442-VWFpp does not traffick to ACTH-containing granules. (B) AtT-20 cells expressing wild-type VWFpp or Δ437--442-VWFpp were incubated with either OptiMEM or 5 mmol L−1 8-Br-cAMP. The amount of released VWFpp was determined by enzyme-linked immunosorbent assay. Wild-type VWFpp was released in response to agonist, whereas Δ437–442-VWFpp was not.