Short-range intracellular trafficking of small molecules across endoplasmic reticulum junctions.

Intracellular trafficking is not mediated exclusively by vesicles. Additional, non-vesicular mechanisms transport material, in particular small molecules such as lipids and Ca(2+) ions, from one organelle to another. This transport occurs at narrow cytoplasmic gaps called membrane contact sites (MCSs), at which two organelles come into close apposition. Despite the conservation of these structures throughout evolution, little is known about this transport, largely because of a lack of knowledge of almost all molecular components of MCSs. Recently, this situation has started to change because the structural proteins that bridge an MCS are now known in a single case, and proteins implicated in lipid trafficking have been localized to MCSs. In the light of these advances, I hypothesize that the endoplasmic reticulum has a central role in the trafficking of lipids and ions by forming a network of MCSs with most other intracellular organelles.