INTRACELLULAR TRANSPORT OF SECRETORY PROTEINS IN THE PANCREATIC EXOCRINE CELL

TLDR

Pancreatic secretory protein transport was investigated to determine whether it is obligatorily coupled to protein synthesis or can be independently regulated, with controls showing newly synthesized proteins normally move to Golgi vacuoles. Guinea pig pancreatic slices were pulse‑labeled with leucine‑3H for 3 min, chased 37 min in cycloheximide‑containing medium to inhibit synthesis, and transport was quantified by measuring radioactivity in the zymogen granule fraction. Even at maximal protein‑synthesis inhibition, transport efficiency remained ~80 % of control and all transport steps, including secretion, were independent of synthesis, demonstrating that transport and synthesis are separable processes.

Abstract

Experiments have been carried out to determine whether intracellular transport of pancreatic secretory proteins is obligatorily coupled to protein synthesis or whether it is a separable process which can be independently regulated. To this intent, guinea pig pancreatic slices were pulse labeled with leucine-(3)H for 3 min and incubated post-pulse for 37 min in chase medium containing cycloheximide up to concentrations sufficient to inhibit protein synthesis by 98%. In controls, newly synthesized secretory proteins are transported over this interval to condensing vacuoles of the Golgi complex. Since the latter are recovered in the zymogen granule fraction upon cell fractionation, intracellular transport was assayed by measuring the amount of protein radioactivity found in the zymogen granule fraction after a (3 + 37) min incubation. The results indicated that at maximum inhibition of protein synthesis (5 x 10(-4)M cycloheximide), transport proceeded with an efficiency approximately 80% of control. Parallel radioautographic studies on intact slices confirmed these data and further indicated that all the steps of intracellular transport, including discharge to the acinar lumen, were independent of protein synthesis. We conclude that: (1) transport and protein synthesis are separable processes; (2) intracellular transport is not the result of a continuous delivery of secretory proteins from attached polysomes to the cisternae of the rough endoplasmic reticulum; and (3) transport is not dependent on the synthesis of "specific" nonsecretory proteins within the time limits tested.