Protein traffic from the secretory pathway to the endosomal system in pancreatic beta-cells

Constitutive-like secretion involves vesicular trafficking corresponding ki netically and biochemically with a post-trans-Golgi network (TGN) origin. I n pancreatic beta-cells, the budding of AP-1/clathrin-coated vesicles, a po rtion of which is derived from immature secretory granules, has been hypoth esized to initiate constitutive-like trafficking. However, similar to 30 mi n after release of a 20 degrees C intracellular transport block in pancreat ic beta-cells (to synchronize protein egress from the TGN), addition of bre feldin A (BFA) (which inhibits AP-1 recruitment) was reported not to block subsequent constitutive-like secretion. To further explore post-TGN traffic king in pancreatic beta-cell lines, we have followed the fate of pulse-labe led procathepsin B (ProB, a lysosomal pro-enyzme) after postpulse wortmanni n treatment or the BFA treatment described above. We find that continuous w ortmannin treatment allows ProB to reach immature secretory granules but in hibits its egress from maturing granules. Remarkably, BFA treatment causes augmented unstimulated secretion of newly synthesized ProB that is not para lleled by insulin. This effect requires a delay of 25-35 min after release from the 20 degrees C block. Further, when ProB delivery to endosomes is in hibited, its BFA-augmented secretion is eliminated. We hypothesize that the constitutive-like pathway involves an endosomal intermediate.