Organelle structure and biogenesis

This Minisymposium highlighted emerging views of organellar dynamics and interactions.

What controls the dynamics of the transitional ER?

Secretory proteins exit from the endoplasmic reticulum (ER) at specialized transitional ER (tER) sites. Benjamin Glick (University of Chicago) described the role of Sec16 in regulating tER organization in the yeast Pichia pastoris. Membrane-associated Sec16 seems to slow ER export. A mutation that redistributes Sec16 to the cytosol relieves this restraint, thereby accelerating both ER export and tER dynamics. Cells may regulate ER export by modulating the level of membrane-associated Sec16.

How do resident ER proteins escape secretion?

For ER resident proteins, one way to remain in the ER is to avoid being exported at tER sites. Abby Vander-Heyden from the Hanson lab (Washington University) described her studies of the luminal ER resident protein torsinA. She showed that torsinA stays in the ER by a process involving a hydrophobic N-terminal helix that partitions into the ER membrane without crossing the bilayer. The working hypothesis is that torsinA is excluded from tER sites through association with specific lipid domains.

How do Golgi stacks regulate membrane traffic?

Secretory proteins that leave the ER enter the Golgi. This organelle is stacked in most eukaryotes, but the functions of stacking are unclear. Yanzhuang Wang (University of Michigan) described how peripheral membrane proteins can mediate Golgi stacking. Surprisingly, inhibition of stacking enhanced vesicle formation and Golgi traffic. Oligosaccharide processing was compromised under these conditions, suggesting that Golgi stacking increases glycosylation fidelity at the expense of speed.

How does ER shape affect ER–mitochondria junctions?

The ER forms contact sites with mitochondria, and this interaction is mediated by the ERMES complex. Contact sites apparently function to exchange phospholipids between the ER and mitochondria. Christiane Voss from the Prinz lab (National Institutes of Health [NIH]) found that ERMES components are required for viability of strains with defects in their reticular ER. The reticular shape of the ER may enable ER tubules to come close enough to mitochondria (or even to wrap around them!) to allow lipid transfer.

How do mitochondrial fission and fusion regulate mitophagy?

Atsushi Tanaka from the Youle lab (NIH) studies the Parkin protein, which is mutated in certain forms of Parkinson’s disease. He showed that Parkin is a ubiquitin ligase for mitofusins, the GTPases that regulate mitochondrial fusion. In damaged mitochondria, ubiquitinated mitofusins undergo proteasomal degradation by AAA+ ATPase p97, preventing the damaged mitochondria from fusing with healthy mitochondria. The damaged mitochondria are then turned over by mitophagy. Therefore, mutations in Parkin may cause disease by promoting the accumulation of damaged mitochondria.

How dynamic are organelle structure and composition?

Organelles change their shape and protein content to cope with a changing environment. Maya Schuldiner (Weizmann Institute) described a new methodology to study yeast organellar dynamics using high-throughput microscopy to measure the localization and levels of proteins in a collection of 6000 yeast strains, each of which contains a different protein fused to green fluorescent protein. Visualizing cells under a variety of growth conditions reveals dynamic changes in protein levels, organelle composition, and organelle shape and size.

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