Thursday, 24 March 2016

Flagella: Molecule(s) of the Month for April 2016

I was scanning my twitter account the other week, when a tweet from a colleague (@jwscattergood) suggested looking at some amazing images from a collaborative structural biology project at Imperial College. The structures of three separate bacterial flagella formed the focus of a publication in the open access journal. Proceedings of the National Academy of Science (follow this link). The structures (shown LHS is the Vibrio fischeri flagellum) produced via cryo electron microscopy (an original image before reconstruction is shown top LHS) in conjunction with applied molecular cloning technologies, are truly stunning and just as the double helix of DNA is iconic to all, the flagella of cells is certainly a "small wonder of Nature" in my book, as well as John's.

The bacterial flagellum is shown opposite (RHS) in a diagrammatic representation and is effectively a rotary outboard motor that propels some, but not all bacteria. Most bacteria that possess a flagellum are rod shaped, and when I asked a microbiology colleague to estimate the proportion of bacteria with flagella, he thought it would be less than half (if anyone has surveyed environmental populations, do let me know). So, this suggests that they are useful, but not essential. In fact in the case of certain Vibrio strains (see here) and additional set of lateral flagella can be "induced" under conditions of nutritional limitation, to help the starving bacteria find food beyond their immediate "neighbourhood". They have also become the focus of a longstanding evolutionary-creationist debate. How can such a complex molecular machine have evolved, if not by the hand of some "divine" creator. Well, in my view the fact that such complex machines (as complex as they are beautiful) are usually assembled from other parts of a bacterial cell, they are a wonder of evolution. It has been demonstrated that  flagella possess fewer dedicated component than they do multi-functional proteins, drawn from other segments of the bacterial genome. You can read the background  to the controversy here (and I recommend Sandwalk for a lively and up-to-date forum for controversies over matters scientific and creationist). It seems that flagella have largely evolved from the secretory apparatus, but I want to return to the structural aspects of these molecules and related ones.

The recent structural studies address the differences in the "power" of different bacterial flagella, demonstrating that additional protein molecules are recruited in those flagella that "navigate" the bug through murkier waters! I like this section from the abstract of the paper by Beeby et al (2016).

For the first time, to our knowledge, our results unambiguously locate the torque-generating stator complexes and show that diverse high-torque motors use variants of an ancestrally related family of structures to scaffold incorporation of additional stator complexes at wider radii from the axial driveshaft than in the model enteric motor.

This could just as well be a group of electrical engineers writing (albeit at the   nanotechnology level): we are beginning to witness the convergence of fields, more than ever before and I think it is incredibly exciting! These ideas and discoveries will surely provide great opportunities and examples for teachers to engage students in the value of multi-disciplinary science!
In eukaryotes, the class of cells that are truly different from all others, but still well known, are the spermatozoa. I am pretty sure that everyone is familiar withe the ability of the  human spermatozoon (LHS) to swim towards the ovum. However, unlike bacteria, the outboard motor of spermatozoa does not rotate. Instead it beats from side to side, propelling the sperm in a less direct manner. The motion and mechanism of sperm flagella is the same as the beating of cilia in a number of eukaryotic specialised epithelia. However, I leave the molecule of this month with these image reconstructions from Beeby et al, which demonstrate the beauty of Nature at the molecular level.