Approaches to Organelle Spacing at Membrane Contact Sites

Membrane contact sites (MCSs) are points where organelle surfaces are juxtaposed, without fusion of their membranes. Research in the MCS field has widely focused on the molecules and mechanisms that hold organelles together, known as "tethers", based on the correct presumption that processes mediated by MCSs depend on their spatial proximity. However, it has become clear that the distance between the membranes also have a biological role, particularly in processes such as Ca²⁺ transfer, while its relevance for other functions, such as lipid exchange, remains less clear. This supports the idea that there may be molecules that keep membranes apart, referred to as "spacers". While some evidence supports this hypothesis, the mechanisms of spacing remain unclear. In this opinion article, we briefly describe three potential spacing mechanisms that we have hypothesised based on direct and indirect experimental evidence. Briefly, we propose the existence of different types of organelle spacing approaches (OSAs) that result in organelle distancing: first, the presence of rod-like spacing molecules; second, the removal of some tethers, achieved through their masking, dissociation from MCSs, or degradation, which could result either in complete untethering of the organelles or in variations in the distance between them due to changes in the composition of the MCSs; third, cytoskeleton-mediated pulling forces that can increase the distance between the two apposing membranes. Our aim is to stimulate discussion and initiate debate on the importance of maintaining the correct distance between organelles.