Summary and interpretation of the
combined data. The model shows development and maturation of ESVs
starting with small tER-derived putative COPII vesicles, which merge to
form larger immature ESVs. Separation of CWP-containing vesicles from
those containing cargo secreted to the plasma membrane is achieved by
selective fusion competency of transport intermediates. Association of
small ESVs with COPI suggests maturation by retrograde transport to the
ER. According to the data presented here, immature ESVs may therefore
represent transient Golgi-equivalents, which develop to
clathrin-associated trans-Golgi-like cisterna. Mature
ESVs become responsive to an internal signal initiating nuclear
division and ESV fragmentation into small carrier vesicles and/or
tubules, which fuse with the plasma membrane. After secretion of the
extracellular matrix, plasma membrane proteins, such as VSPs, are
internalized and presumably transported to the lysosome-like PVs for
recycling.