Sorry for the lack of posts. I've been reconstructing the figures and the text of my manuscript and expect to resubmit the darn thing to PLoS Biology later this week. Until then here is an interesting paper that I've read recently:
So what's this all about? Well the main player is Atg8, a protein that is essential for forming large multi-vessicular bodies (or MVBs).
What was that?
When cells are under starvation conditions they start to consume themselves - this process is called autophagy (self eating). Cells collect organelles and miscellaneous items into these huge bilayered organelles or MVBs. These MVBs then fuse with lysosomes, the trash compactors of the cell. Lysosomes are membrane bound organelles that are filled with enzymes that breakdown the proteins membranes and polycarbohydrates. Thus using this system, the cell can eat its own mitochondria, protein aggregates, miscellaneous vesicles etc...
Now it has been a mystery how a bilayered membrane can form presumably from smaller vesicles ... but in this paper the authors demonstrate that Atg8, a ubiquitin like molecule, can form oligomers that promote the fusion of vesicles. The idea is that as two vesicles get close to eachother Atg8 may form a connecting ring that promotes the mixing of the outer leaflets of the neighboring vesicles. An example of this process shown on the EM micrograph (right) where two vesicles are shown in the process of fusion. Atg8 proteins, which are detected with immuno-gold particles, accumulate between the two vesicles.
The idea is that small vesicles use Atg8 oligomers to fuse and thus form these giant sheets that eventually become the outer bilayer of an MVBs.
Here's a schema that I stole from a preview by Suresh Subramani, and Jean-Claude FarrÃ©:
Now the big questions are 1) how would a ubiquitin-like molecule oligomerize? and 2) why would this promote membrane fusion?
And 3) if this hemifusion intermediate is actually an intermediate on the pathway, how does the transition to full fusion occur?
I'm smiling because in 1999/2000 I wrote a final research paper (undergrad honors work) on the proteinaceous pore/hemifusion intermediate controversy ... and the jury is still out!
->1) ubiquitin itself is not very prone to organized oligomerization. On the other hand Atg8 is only distantly related to ubiquitin. There are ubiquitin-fold proteins that are known to oligomerize, e.g. the PB1 domain family. I am not sure about Atg8, but you should expect anything from a ub-like protein that becomes conjugated to a lipid...
That whole hemifusion business is kinda strange. Such structures are never seen in vivo.
In our lab, we just assumed that their in vitro reconstitution is not efficient or that there is some component that is missing from the process. But yes as in other fusion processes, how hemifused membranes are transitioned to fully fused membranes remains murky.
I was looking for some Atg8 info on line and found this interesting article. Any answers on those two big questions?