In addition to their extensive repertoire of membrane-embedded chemoreceptors, cells may also have another type. Many species of bacteria and archaea, like this Methanoregula formicica, contain cytoplasmic chemosensory arrays. The receptors lack membrane insertion patches. Instead, they interact end-to-end with each other at their chemical-sensing tips, zippering into a sandwich with a baseplate of kinases and coupling proteins on either side. In some species, the arrays are curved like this; in others, they are straight. What these systems sense remains a mystery. One idea is that they monitor the internal state of the cell (e.g. the levels of various metabolites) to fine-tune signaling responses to meet the current needs of the cell. They are often found along with, and often quite close to, membrane-embedded chemosensory arrays (⇩).
This Vibrio cholerae cell showcases a full complement of chemosensory systems: an extensive membrane-embedded array (signaling to the flagellar motor) and a cytoplasmic array (signaling to a still-unknown target). In each case, the basic architecture is identical: hexagonally-arranged trimers of chemoreceptor dimers.
