Many key biological processes like receptor recycling, deactivation and degradation, antigen presentation, autophagy, and the elimination of microbial pathogens, to mention a few, involve dynamic, often extreme restructuring of the intracellular membranes. These complex tasks are performed by a molecular sorting machinery in combination with coat proteins and force-generating motors that drive membrane deformation and extension. As a result, selected components are delivered to vesicles and tubules that are vectorially directed to intracellular destinations encoded by determinants exposed on their cytosolic surface.
So often, we tend to ignore how these dynamic events are tightly coordinated by the luminal contents (i.e. fluid) of membrane-bound organelles. In immune cells that turnover the equivalent of their entire surface membrane and engulf the volumetric equivalent of their cytosol in a matter of hours, fluid traffic cannot be ignored – in fact, it can be readily studied. I will discuss our recent findings that describe how the transport of ions and organic solutes in endosomes and lysosomes control membrane traffic. The molecular mechanisms are scaled to the tissue level to understand the importance of fluid traffic in the immune response to cellular damage.