The shaping of surface clusters

A new study describes how active and passive exchange of cellular material controls the formation of membrane clusters.

• The exchange of molecules between the cell and its membrane controls the size and distribution of clusters formed on the membrane
• passive and active exchange can accelerate coarsening, the formation of fewer and larger clusters over time. Active exchange can alternatively also suppress coarsening
• The model applies to various biological systems including bacteria, synapses and model organisms where pattern formation is observed

Cellular membranes do not only constitute a barrier, but they accommodate also a plethora of different molecules for sensing and to ensure cellular function. These molecules often cluster together forming condensates embedded in the membrane. A team of physicists from MPI-DS investigated how the exchange of material with the interior of the cell affects the size and number of such clusters. They showed in their model that the coarsening of the clusters – the formation of fewer and bigger structures over time – can depend on the type of exchange between the membrane and the bulk. If this exchange takes places passively, it accelerates the coarsening process.

Surface clusters are observed for example in the case of receptors on the plasma membrane, which come together to improve signaling,” explains David Zwicker, group leader at MPI-DS and last author of the study. Such clustering of molecules is a common phenomenon in cellular biology where phase separation can initiate cellular processes. In the simple case where exchange takes place passively, such cluster formation leads to coarsening, which can have cellular function as previously described. In contrast, coarsening can also be caused by enzymatically driven, active exchange, which can accelerate coarsening. An example for this is the formation of polarity spots in budding yeast where proteins are concentrated to define where a new bud will emerge. “Our model provides an explanation how this very fast recruitment process of proteins can be achieved,” comments Riccardo Rossetto, first author of the study.

Moreover, active exchange is not only able to accelerate coarsening, but can also inhibit it, according to the model. If membrane receptors in single cell organisms such as bacteria all would accumulate at one location, the sensing of the surrounding would be limited to this spot. Thus, the patches or units of receptors need to be distributed across the membrane while still allowing dynamic rearrangement. Here, active exchange makes it possible to control the cluster size and distribution.

The findings of the study help to explain how membrane patterns emerge and take shape in biology. The underlying model applies to a variety of biological systems, including bacteria, neuronal synapses and model organisms.