Endoplasmic reticulum-endosome contact increases as endosomes traffic and mature

• Published in: Molecular biology of the cell 2013-04, Vol.24 (7), p.1030-1040
• Main Authors: Friedman, Jonathan R, Dibenedetto, Jared R, West, Matthew, Rowland, Ashley A, Voeltz, Gia K
• Format: Article
• Language: English
• Subjects: Animals; Cell Line, Tumor; Chlorocebus aethiops; COS Cells; Endoplasmic Reticulum - metabolism; Endoplasmic Reticulum - ultrastructure; Endosomes - metabolism; Endosomes - ultrastructure; Humans; Luminescent Proteins - genetics; Luminescent Proteins - metabolism; Membrane Proteins - genetics; Membrane Proteins - metabolism; Microscopy, Confocal; Microscopy, Electron; Microtubules - metabolism; Microtubules - ultrastructure; Multivesicular Bodies - metabolism; Multivesicular Bodies - ultrastructure; Nocodazole - pharmacology; Protein Transport - drug effects; rab GTP-Binding Proteins - genetics; rab GTP-Binding Proteins - metabolism; rab5 GTP-Binding Proteins - genetics; rab5 GTP-Binding Proteins - metabolism; rab7 GTP-Binding Proteins; SEC Translocation Channels; Time-Lapse Imaging
• ISSN: 1059-1524; 1939-4586
• DOI: 10.1091/mbc.e12-10-0733

The endosomal pathway is responsible for plasma membrane cargo uptake, sorting, and, in many cases, lysosome targeting. Endosome maturation is complex, requiring proper spatiotemporal recruitment of factors that regulate the size, maturity, and positioning of endosomal compartments. In animal cells, it also requires trafficking of endosomes on microtubules. Recent work has revealed the presence of contact sites between some endosomes and the endoplasmic reticulum (ER). Although these contact sites are believed to have multiple functions, the frequency, dynamics, and physical attributes of these contacts are poorly understood. Here we use high-resolution three-dimensional electron microscopy to reveal that ER tubules wrap around endosomes and find that both organelles contact microtubules at or near membrane contact sites. As endosomes traffic, they remain bound to the ER, which causes the tubular ER to rearrange its structure around dynamic endosomes at contact sites. Finally, as endosomes transition through steps of maturation, they become more tightly associated with the ER. The major implication of these results is that endosomes mature and traffic while coupled to the ER membrane rather than in isolation.