by Christoph
Figure 1. Detail from Alise Muok's image. Source
The same frontispiece twice within a few days at STC? No, not quite. It's not about Roberto's hitchhiking phage this time, or the flagellated "night green" bacterium on the right in Figure 1, or the hitchhiking spore (umber-brown, in the center), but about the tip of the flagellum (black, on the left). Disclosure: already back in 2020, I talked with Alise, who created the image, and three other passionate science illustrators, Liza, Noémie, and Eliza about the issue with the flagella tip (like STC, Lizah, Noémie, and Eliza are also on mastodon now). This was on Twitter, so now here's a little more in-depth discussion (can you spot the flagellated bacterium on this recent poster by Lizah? ha!).
What is so troubling about this depiction of flagellar tip? Of course, it is obvious for a drawer to indicate the end of a line by tapering it, by simply applying less pressure to the pen or lifting the brush slightly. Flagella are in fact quite flexible and sometimes appear in electron micrographs as loops without breaking (see here a flagellated Proteus mirabilis cell, a highly motile bacterium), but they have the same diameter over their entire length, that is, without a visible thinning towards the tip. A good analogy from the macroscopic world is a garden hose with one end connected to the faucet and the other end equipped with a nozzle.
Figure 2. Wikipedia: Structure of bacterial flagellum (cropped). Source
The thinning of the flagellar filament is even more extreme in Figure 2. Here, the illustrator made use of a common trick to accommodate the flagellar filament, which is significantly longer than the cell with the membrane-anchored motor, in one picture: foreshortening in perspective. Even though this might be obvious even to a naive viewer of this graphic, another aspect is really misleading: the filament thus appears as if continuously extruded from the flagellar hook, tip first. Nothing could be more wrong! (admittedly, such a misleading illustration can also be found in STC here.)
Figure 3. A schematic plot of the major structural components of the bacterial flagellum. Source
The schematic plot of the major structural components of the bacterial flagellum shown in Figure 3 completely forgoes perspective and only hints at the length of the filament through a (directional) interruption. More importantly, this plot correctly depicts the successive build-up of the filament: stretched-out monomers of the flagellin (=filament subunit protein) FliC are pushed through the inner channel of the motor, the hook, and the forming filament up to the tip, where the cap protein, a FliD pentamer, 'chaperones' FliC into its final conformation and position. A process that resembles the building of a chimney where the individual bricks are transported up inside.
What happens when the capping protein FliD is missing from the filament assembly has been shown very clearly for Borrelia's periplasmatic flagella by Zhang et al. (2019). In a fliD mutant, the FlaB flagellin subunits are transported through the hook into the periplasm, but cannot polymerize properly there and are (proteolytically) degraded by HtrA (see Figure 4). What happens at the flagellar tip, that is, how the flagellum is successively elongated by incorporating flagellin subunits just underneath the pentameric cap protein complex and with its help, has been modeled by biophysicists (Figure 5). This evocative model could/should serve scientific illustrators and cartoonists as a suitable template for the design of flagellar tips.
Figure 4. Graphical abstract. Source
I am a passionate believer in artistic freedom, no question. I can relate to the attraction that moved Chinese artist Ai Weiwei to recreate Claude Monet's famous water lilies – see here the MoMA version – with Lego bricks – see here. Whether the result is captivating is something everyone can decide for themselves, a matter of taste. Artistic freedom should also apply to scientific illustrations, which are extremely important for conveying context that does not fit well in a diagram or table and is more confused than illuminated by photographic images. That said, technical limitations of the illustrator's tools – brush, pencil, digital pen – should not lead to depicting morphological details of cells, let alone molecular structures, contrary to knowledge of their structure.
Figure 5. Cap assists flagellin to fold into place at tip. Animation by Namba Protonic NanoMachine Project. Source
The relevance of the latter can be illustrated (pun intended) by two examples. First, the tips capping filaments of constant diameter along their entire length like in flagella are important in type IV pili (T4P) that bacteria use to reach out for, attach to, and bring closer by retraction of target molecules (see here a Vibrio using its T4P to 'harpoon' DNA for uptake), or conjugation partners, or surfaces for twitching motility. Second, and similar to type IV pili, the effector proteins that type III secretion systems (T3SS) literally inject into target cells sit at the tip of filaments of constant diameter. Thus, any graphics showing successively tapering filaments would therefore be just as wrong as one that emphasizes the filament tips like stone age spearheads.
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