The biosynthesis of fibroin. I. The intracellular form of silk fibroin of Bombyx mori L

After in vivo labeling with ³H]glycine the synthesis and transport of fibroin has been studied by radioautography and cell fractionation.Radioactivity appearing in the cytoplasm is rapidly transferred to the lumen where it accumulates in the so-called silk layer before reaching the central core of secreted fibroin. By sucrose density gradients it was demonstrated that the radioactivity appears immediately in the fibroin fraction, no precursors being observed.A simple fractionation procedure, based on the utilization of detergent, gives three fractions tentatively interpreted as synthesis, transport, and accumulation compartments in accordance with their kinetics of labeling.