ER domains

On page 897, Luedeke et al. show that, like the plasma membrane, the ER can be compartmentalized into distinct membrane domains. In budding yeast, this separation restricts the diffusion of ER membrane proteins between the bud and the mother cell.

The mRNAs of many bud-specific membrane proteins are actively transported into the bud. To keep these mRNAs and proteins in the bud, they must be kept from diffusing back into the mother cell through the ER, which traverses the bud neck. Using FRAP analyses, the authors show that proteins are kept in the bud via a diffusion barrier at the bud neck, where rough ER is replaced by a band of smooth ER.

The lumen of the ER was continuous, as shown...