Nagaya et al. report. The researchers were the first to observe individual proteins moving in the ER.
Freshly made proteins fold into shape as they travel through the ER. But if proteins run into one another before they finish the job, they can end up misshapen or stick together to form potentially toxic aggregations. Chaperones inside the ER, such as the lectin calnexin, help some proteins avoid this fate. But Nagaya et al. wanted to test whether the ER also has a mechanism to put the brakes on speeding proteins.
The researchers found that they could stall movement of some proteins in the ER by exposing cells to a hyperosmotic solution. Molecules that got stuck usually sported oligosaccharides called N-glycans, suggesting that these attachments help slow the proteins down. To get a closer look at events inside the ER, the team turned to total internal reflection fluorescence microscopy. Researchers had previously applied this technique to observe molecules on the cell surface, but Nagaya et al. were able to use it to track proteins moving through the ER just beneath the cell membrane. Instead of progressing smoothly through the tubules, the proteins appeared to be catching. However, they didn't seem to be getting stuck in the ER exit sites.
The researchers also discovered that this slowing required actin but not microtubules. That finding suggests that the actin cytoskeleton exerts control over movements of proteins in the ER. Nagaya et al. hypothesize that lectins cluster on the inner wall of the ER and then grab a passing protein's N-glycans, temporarily detaining it. Actin might help the lectins congregate by corralling them or by serving as a platform where they can gather.