Continuously variable lipid packing as the principle of functional membrane heterogeneity

Lipid rafts are nanoscale entities in the membranes of eukaryotic cells which provide a mechanism for the functional membrane segregation vital for several cellular processes. This lateral segregation of specific lipid and protein components provides the facilitative platforms for a variety of signaling and trafficking events at the plasma membrane and in the Golgi. Rafts are distinguished from the surrounding membranes by their physical properties and composition - they are relatively tightly packed and enriched in saturated lipids, sterols, and lipid-anchored proteins. Although the existence of rafts has been conclusively confirmed by several independent techniques, questions concerning various aspects of membrane heterogeneity are still to be addressed. Typical experiments investigating raft composition have been designed to evaluate the affinity of a given component for raft domains. In such experiments, the results are usually interpreted in a Boolean fashion, i.e., the component is either a raft molecule, or not. However, this binary point of view overlooks potential complexity that may underlie the nature of membrane heterogeneity. In this work, we systematically investigated the nature of functional cellular membrane heterogeneity. We started by characterizing the model membranes and fluorescent lipid analogs widely used in research into membrane domains. After extensively evaluating the potentials/limits of these approaches and the artifacts that must be avoided or alternatively could be exploited, we applied these tools to understand whether the cell membrane has multiple kinds of raft domains with distinct compositions and physical properties, rather than only one. We found that cell membranes have the potential to form various kinds of functional domains having different physicochemical properties, compositions, and functional outputs. Therefore, we propose continuously variable lipid packing as the principle of the functional membrane lateral heterogeneity. According to this principle, the membrane is not composed of a single variety of raft domain with strictly defined properties coexisting alongside a specific and uniform non-raft environment; rather it is composed of entities having continuously variable lipid packing. Finally, we show that this spectrum of membrane packing modulates the orientation of membrane lipid receptors, which ultimately influences their specific bioactivity. Our results showing continuously variable lipid packing and its ability to fine-tune the activity of membrane molecules comprise a novel model for the structure and function of eukaryotic membranes