General principles of cellular communication

# 3.2 Proteins perform a variety of functions in cell signalling

The proteins involved in signalling can be roughly grouped into a number of categories, based on their functions and activities. Note, however, that the designation of proteins in these categories is not always simple because many proteins within signalling pathways can have more than one function. Some of the ways in which signalling proteins function in signalling pathways are summarised below.

## Box 1 Functions of proteins in cell signalling

Relay proteins: These pass a signal on to the next member in the chain. Many different proteins fulfil this role.

Amplifier proteins: These increase the strength of signalling. To achieve this, they either activate multiple downstream targets, or generate large numbers of messenger molecules. Amplifier proteins are often enzymes, such as kinases, or ion channels that can transport many ions in response to a single activation signal.

Transducer proteins: These change a signal into a different form. For example, a kinase might be switched on by allosteric interaction with an upstream molecule, but subsequently activate a downstream protein via phosphorylation.

Bifurcation proteins: These branch a signal to different signalling pathways.

Integrator proteins: These receive signals from different pathways and integrate their input into a common signalling pathway.

Modulator proteins: These regulate the activity of proteins within a signalling pathway.

Anchoring proteins: These tether members of the signalling pathway in particular subcellular locations, such as the cell membrane or the cytoskeleton, thereby ensuring that the signal is being relayed to the right place.

Adaptor proteins: These link one signalling protein with another. Adaptor proteins usually do not act as a signal themselves, but link signalling proteins together so that information can be relayed along a pathway.

Scaffolding proteins: These bind several signalling proteins and may also tether them, forming a much more efficient functional complex. Scaffolding proteins may therefore share attributes of both anchoring and adaptor proteins.

Based on the descriptions of different signalling protein functions in Box 1, attempt to interpret the roles of the proteins in a hypothetical signal transduction cascade in the activity below.

## Activity 7 Signal transduction cascades

Allow 10 minutes

Consider the hypothetical signal transduction cascade depicted in Figure 10. Different types of signalling protein are listed to the right of the diagram. Drag and drop the labels for protein types into their correct positions on the figure. (You can also scroll through the options and make your choices by repeatedly clicking on or tapping a label field on the figure.)

Note: You can obtain hints on the role of any of the unlabelled proteins by clicking or tapping on them; the hint is displayed at the top of the figure.

Figure 10 A hypothetical signal transduction cascade

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