Fusing branes

For the first time this semester, yesterday, I had time to attend our school's colloquium. Everybody in academia will know that there is always a temptation to skip colloquia as they typically are not in your own area of expertise (and often that means interest) and if the speaker is not too good there is often not much you get out of it than at best some intellectual entertainment.

Still, my general philosophy is that in the long run it pays to invest 1.5 hours per week into broadening one's perspective and to get an idea what's hot in other areas of science. Physicists that refuse to spend time with anything else than their own little area of research are in my eyes just ignorants. Nevertheless, I usually take something to read with me so I have a chance to do something slightly more useful with my time once the speaker loses me. In recent times, I have also taken sudokus to seminars but I should better not admit that.

Yesterday, however, there was no need to bring anything, even though the talk was on microbiology because it was just excellent. The speaker was Reinhard Jahn and he talked about fusing membranes.

I have an interest in those as well, at least if we are talking about D-branes:



There, 'recombination' is an elementary process in which two branes join into a single one. It is still simple enough to be understood in detail but via dualities it is related (or archetypical) to many topological transitions (like small instantons, M5-branes falling into end-of-the-universe branes in heterotic M-Theory/Ekpyrotic scenarios etc) in string theory.

In biology as well, one can worry about the microscopic workings of this resolution of a singularity. Different from string theory, cell biology has a natural discreteness at small scales and at some point it becomes important that biological membranes are double layers of peptides sticking together by hydrophobicity.

So, I learned that cell membranes are not really elastic and if a cell (or one of its compartments) grows it has to add membrane which comes in small spherical blobs that fuse with the membrane that is about to grow:



Reinhard Jahn explained the inner workings of the process. An important role is played by somewhat longish specific proteins (SNAPs) that have one end sticking into the membrane and an alpha helix sticking out into the surroundings. Now, if two membranes get near each other, the helices of four of these SNAPs can coil up and form a highly stable supercoil. This draws the membranes together so they can fuse as well as releasing binding energy in the coiling up process that is used to overcome an activation energy barrier for the fusing process. Very fascinating and brilliantly presented!