Chapter 22

Biological Signalling

The term “biological signalling”, frequently referred to as “signal transduction”,

“molecular signalling” or simply “signalling” mostly refers to the process whereby

information about a cell’s environment is transferred to the nucleus (of a eukary-

otic cell), where it affects changes in gene expression appropriate to changes in the

environment. The process is captured in Fig. 7.1. Biological signalling also refers

to nervous signalling (Chap. 24) and metabolic communication via hormones, both

within an organism. ¹ In the prokaryotic world, there is no nucleus but information

about a cell’s environment is similarly transferred into the cell interior to modulate the

activity of nuclear transcription factors, thereby regulating gene expression. Bacteria

can also communicate via “horizontal” gene transfer, whereby fragments of DNA

from a bacterium are taken up by other bacteria, much as a viral genome can become

incorporated into a bacterial one. Hence, although signalling mainly concerns onto-

genetic adaptation (and behavioural adaptation in the case of nervous signalling), it

also encompasses phylogenetic adaptation. This chapter is essentially devoted to cell

signalling, which is the most fundamental process: neurons function using it, and

hormones require receptors for transducing the signals they convey into a form that

the cell can “understand” and use to actuate gene expression (or the suppression of

expression).

Despite the simplicity of Fig. 7.1, the molecular processes are intricate and rami-

fied. ² The process starts on the cell membrane, where specialized membrane receptors

selectively detect and bind ligands, providing an information filter; this step is some-

times called “molecular recognition”. Conformational changes in the receptor ensue.

Typically the receptor molecule traverses the membrane, and conformational changes

on the cytoplasmic side may alter enzymatic activity (such as phosphorylation—cf.

the kinome, Sect. 18.7), or the binding of “mediators” (small molecules that can alter

the activity of other enzymes); ultimately transcription factors are affected, changing

the pattern of gene expression. Some transmembrane receptors act as ion channels

1 Individual cells can communicate with their nearest neighbours via the so-called gap junctions.

2 Krauss (2008), Péter et al. (2021).

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