I bet a lot of you have that ex-girlfriend or ex-boyfriend who just hangs on….and hangs on tighter even though you’ve driven off to college and totally matured past the desire to want someone who can crush a beer can on his/her forehead. He/she was totally pulling an endoplasmic reticulum, and the paper that brought us today’s image shows us why.
Endosomes form at the plasma membrane, where they take in material from outside the cell. They mature, with some cargo recycled back to the plasma membrane and some cargo trafficked to lysosomes for degradation. The cytoskeletal tracks (microtubules) that endosomes depend on for movement around the cell are also used as tracks for the endoplasmic reticulum (ER), where proteins are synthesized, folded, and sorted. Recent work has shown interactions between endosomes and the ER, and a more recent paper characterizes these interactions. Friedman and colleagues used high-resolution three-dimensional electron microscopy and found that ER tubules wrap around maturing endosomes, while both organelles maintain contact with microtubules. As the endosomes mature, they become more tightly connected to the ER. Similarly, as an endosome traffics, the interacting ER rearranges its structure in order to maintain endosome contact. In the images above, early endosomes (Rab5, red) are tightly associated with ER membranes (green). Higher magnification images (bottom) of the boxed region show three early endosomes (numbers 1,2, and 4) that are and remain in contact with the ER over the course of two minutes, and one endosome (number 3) that becomes associated.
Friedman, J., DiBenedetto, J., West, M., Rowland, A., & Voeltz, G. (2013). Endoplasmic reticulum-endosome contact increases as endosomes traffic and mature Molecular Biology of the Cell, 24 (7), 1030-1040 DOI: 10.1091/mbc.E12-10-0733