West et al. reveal the diversity of endoplasmic reticulum (ER) structures present in yeast and describe how these structures pass from the mother cell into the bud.
The peripheral ER is a continuous network of membrane tubules and cisternae that extends from the nuclear envelope. West et al. looked at ER structure using electron tomography to reconstruct the organelle's three-dimensional organization.
The researchers saw three types of ER. ER tubules branched from the nuclear envelope throughout the cytoplasm, connecting distinct ER domains and contacting other organelles. A single central cisterna stretched from the nuclear envelope toward the developing bud. The rest of the ER was tightly associated with the plasma membrane as a network of both tubules and cisternae. Plasma membrane–associated ER was unable to enter the bud directly. Instead, the central cisternal and tubular ER domains extended into the bud and then spread out to re-associate with the daughter cell cortex.
ER membranes are partly shaped by two protein families called reticulons and Yop1. Yeast lacking these proteins lost all curved ER structures, with most of the organelle forming a single flat cisterna close to the plasma membrane. Yet curved ER tubules were still pulled into mutant buds before flattening back out, indicating that reticulons and Yop1 are required to maintain, but not generate, ER membrane curvature. Author Gia Voeltz now wants to investigate the ER's contacts with other organelles, particularly its extensive association with the plasma membrane.