Similar to flagella in bacteria, different archaeal species employ different numbers and patterns of archaella. Some species have one, others have many, either distributed peritrichously (all around) as in the Methanoregula formicica you just saw, or lophotrichously (clumped) as in this Thermococcus kodakaraensis. In T. kodakaraensis and related species, an additional structure–a large conical plate–is seen in the cytoplasm, perhaps providing leverage for the multiple motors. The plate has a unique structure (⇩) and may act as an organizing center akin to the polar PopZ structure we discussed in the last chapter (⇩). Note the two peaks on this cone; it may be in the process of replicating in preparation for division.
A leveraging plate must not be essential, however, because not all lophotrichous archaea use one (⇩).
The archaellar plate does not come to a point at the tip, but rather is a conical frustum (open at the top), resembling a lampshade. In the center of the tip is a small ring, as you can see in a top view in this lysed, and therefore flattened, Thermococcus kodakaraensis cell. The function of the ring remains unknown; perhaps it nucleates the rest of the structure?
As you can see more clearly in this lysed, flattened Thermococcus kodakaraensis cell, the conical plate in the cytoplasm is attached to more than just the archaella. It is also associated with chemosensory arrays (discussed in the next chapter) and DNA, as you can see from the ribosome-excluding zone. This structure may therefore perform an analogous function to bacterial organizing proteins such as PopZ, tethering cellular components into a de facto pole for the (in this case round) cell.
While a cytoplasmic plate may help distribute the force of multiple, closely-packed archaella, it is clearly not necessary since other lophotrichous species, like this Halobacterium salinarum, do not use one.
