A study to be published later this week in the Journal of Biological Chemistry reports the identification of proteins that allow the mitochondria and the endoplasmic reticulum to attach to each other.
"Think of an organelle like a ferry docking at one site, unloading and loading passengers and cars, and then going to another site and doing the same thing," said Jeffrey Golden, a professor at Brigham and Women's Hospital and Harvard Medical School who oversaw the work. "Their ability to dock, load, and unload cargo requires guides or ramps of specific width and heights that connect the boat and land or they cannot freely load and unload."
Contact points between the endoplasmic reticulum (ER) and mitochondria are those "ramps" and "guides" that enable these contacts. They permit important activities like signaling, exchange of calcium and lipids, and control of mitochondrial physiology.
Golden's collaborator Ginam Cho and research fellow Il-Taeg Cho set out to find proteins important for ER-mitochondrial contact. They used ascorbate peroxidase, or APEX, which can attach biotin to proteins nearby. The team engineered cells to produce mitochondria that had APEX attached to their outer membranes, and then added biotin to the cells for the APEX to use to label nearby proteins.
The team then isolated parts of the cell that contained the ER, purified those proteins that had biotin attached, and identified the ones found in the ER using mass spectrometry. Because the APEX was attached to mitochondria, only those proteins that came into close proximity to the mitochondria could have had biotin attached.
"It was previously feasible to only look at one molecule at a time to assess what it interacted with," Golden said. "The method we have used is more rapid and allows an unbiased look at a whole system and what's happening at that organelle's interface."
Using this screening method, the researchers zeroed in on an ER protein called RTN1a, which was previously known to contribute to the ER's shape. In follow-up experiments, they confirmed that this protein also helped mitochondria to attach to the ER.
This study raises the possibility that defects in RTN1a could contribute to the problems experienced by patients with neurodegenerative diseases, but the researchers won't know for sure until they conduct additional experiments including similar studies in neural cells.
Golden speculates that the proteins important for ER-mitochondrial contact might be different in different cell types.
The team is now using the APEX-mass spectrometry method to compare proteins involved in ER-mitochondrial contacts between normal and patient-derived neural cells.
Image: Live-imaged HeLa cells with the endoplasmic reticulum labeled red and mitochondria labeled green. Image courtesy of Ginam Cho.