On the cell picture
Cell motility
The cell picture shows a flagellum, the whip-like tail that Aquifex aeolicus uses to swim. The minimal cell moves and holds on differently: with a type IV pilus, a thin filament it extends, attaches to a surface and pulls back in.
In the minimal cell: the type IV pilus
Six proteins of the 1,051 build and run the type IV pilus. With it a cell attaches to surfaces and moves by twitching: the filament reaches out, sticks, and is pulled back in. In flowing hot-spring water this lets a cell hold its place or move to better conditions. Both free-living relatives used as benchmarks, Hydrogenobacter thermophilus and Thermocrinis albus, keep a one-to-one counterpart of every one of these proteins.
| Gene | Protein | What it does | Records |
|---|---|---|---|
| ppdD1 | Pilin | The main building block: many copies stack into the pilus filament. | O67423 ↗ |
| pilD | Type 4 prepilin peptidase | Cuts the leader sequence off each pilin so it can be built into the filament. | atlas page · O67535 ↗ |
| pilC1 | Fimbrial assembly protein PilC | Inner-membrane platform on which the filament is assembled. | atlas page · O66951 ↗ |
| pilC2 | Fimbrial assembly protein PilC2 | A second platform protein of the same kind. | O67317 ↗ |
| pilT | Twitching motility protein PilT | ATP-driven motor that pulls the filament back in, dragging the cell forward. | O66950 ↗ |
| pilU | Twitching mobility protein PilU | A second retraction motor working with PilT. | O67906 ↗ |
Why the flagellum is not in the minimal cell
Aquifex aeolicus VF5 carries 26 flagellar genes. Thermocrinis albus keeps one-to-one counterparts of 23 of them; Hydrogenobacter thermophilus keeps none. A free-living cell with the same way of life manages without a flagellum, so the flagellum is not part of the minimum. The flagellum on the cell picture shows Aquifex itself, not the minimal cell.
Open problems and sources
The motility chapter of the books (Vol. III, Ch. 9, from p. 361) discusses early motility but lists no conceptual problems of its own, so none of the 1,976 open problems is specifically about motility.
How proteins were placed in the 1,051: How the working minimum was reached. Protein records: UniProt proteome UP000000798.