Golgi Maturation

No soccer today. So instead of spending time watching others run around, go read the two papers, published in last week's Nature on Golgi maturation.

Proteins that need to traverse, or be embedded within membranes are synthesized in the endoplasmic reticulum (ER) and are transported cotranslationaly for the most part through a pore called the translocon. Most of these proteins are then delivered, through vesicular transport to the Golgi complex, where they are post-translationally modified. Sugars are added, sugars are removed ... indeed these proteins undergo many different modification steps. The Golgi is divided into many pancake shaped organelles stacked on top of each other, every pancake (or cisterna) having a different modification enzymes. Like penalty kicks and red cards, the Golgi seems to be an endless source of controversy. Does the Golgi dissolve into the endoplasmic reticulum during mitosis? Do proteins travel via vesicular transport from cisterna to cisterna, or does each cisterna "mature"?



But first, what does "mature" mean? It is the act of replacing one set of modifying enzymes with another set of modifying enzymes. So that early (or cis) Golgi is gradually transformed into Golgi containing the next set of modification enzymes (medial Golgi) which in turn is gradually transformed into late (or trans) Golgi. Well actually there are many cisterni (in the neighborhood of 6) in each Golgi stack. From the Golgi vesicles full of modified proteins are transported to various intracellular compartments such as the endosome recycling compartment.

How to tell whether Golgi cisterni "mature"? you would think that it's easy. Just fluorescently label proteins that traverse the Golgi and watch whether they stay in one cisterna or whether they travel across a stack. Unfortunately the stacks are smaller than the resolution limit of light microscopy. One could also image Golgi modification enzymes, but the size issue remains. Another attempted method, is to load the Golgi with giant particles. These substrates being too large to fit in vesicles, would presumably be stuck in one cisterna. But such experiments never gave conclusive evidence.

So what to do? Two groups solved this by reverting to yeast. (It seems like yeast are just simply the best organism to get at tricky problems ... ) You see yeast are weird. For a long time it was even questioned whether they had Golgi as no Golgi stacks were ever seen by electron microscopy. It turned out that the lack of stacks was due to the unique organization of the Golgi in yeast. You see in yeast the different cisterni are physicaly separated ... there are no stacks. And that is no problem for budding yeast, cuz they are so damn small. Also Golgi organization in "higher eukaryotes", is dependent on the microtubule network, but yeast do not use microtubules to direct vesicular transport. Now most proteins that are important for Golgi function are conserved between yeast and other eukaryotes, so even if ultra-structurally the Golgi shape may vary, the core operating system should be conserved.

OK fine so lets use yeast and image each separate Golgi sac to see if each sac matures, or to see whether substrates jump from sac to sac. To answer this, both groups labeled early and late Golgi markers and imaged the distribution of these markers over time. Do the markers jump from cisterna to cisterna? Or is each cisterna static in it's composition of modification enzymes.

So the answer: Golgi stacks mature. The rate of Golgi maturation is compatible with the rate of secreted protein production. If you block vesicle transport (via COP I temperature sensitive mutation) Golgi maturation slows down, indicating that modification enzymes get transported to and from each cisterna ... but since cistetna still matured, other transport mechanisms may help "mature" any particular Golgi sac.

OK that's all I have to say. Go check out the papers, and watch the movies. The results are pretty clear, hopefully the game on Sunday will provide for as much excitement.

Refs:

Eugene Losev, Catherine A. Reinke, Jennifer Jellen, Daniel E. Strongin, Brooke J. Bevis and Benjamin S. Glick
Golgi maturation visualized in living yeast
Nature (06) 441:1002

Kumi Matsuura-Tokita, Masaki Takeuchi, Akira Ichihara, Kenta Mikuriya and Akihiko Nakano
Live imaging of yeast Golgi cisternal maturation
Nature (06) 441:1007

Crossposted at The Daily Transcript.