Clathrin and post-Golgi trafficking: a very complicated issue

• Four papers have been published that relate to the function of TGN-derived CCVs.
• Vacuolar protein transport in μ-adaptin mutant plants is seriously impaired.
• Secretion and SNARE transport to the cell plate are also affected in the mutants.
• These data do not support a role for CCVs in vacuolar protein transport.

Clathrin-coated vesicles (CCVs) are formed at the plasma membrane and act as vectors for endocytosis. They also assemble at the trans-Golgi network (TGN), but their exact function at this organelle is unclear. Recent studies have examined the effects on vacuolar and secretory protein transport of knockout mutations of the adaptor protein 1 (AP1) μ-adaptin subunit AP1 M, but these investigations do not clarify the situation. These mutations lead to the abrogation of multiple trafficking pathways at the TGN and cannot be used as evidence in favour of CCVs being agents for receptor-mediated export of vacuolar proteins out of the TGN. This transport process could just as easily occur through the maturation of the TGN into intermediate compartments that subsequently fuse with the vacuole.