Light on fluorescent lipids in rafts: a lesson from model membranes.

Tracking fluorescent lipids in cellular membranes has been applied for decades to shed light on membrane trafficking, sorting, endocytosis and exocytosis, viral entry, and to understand the functional relevance of membrane heterogeneity, phase separation and lipid rafts. However, fluorescent probes may display different organizing behaviour from their corresponding endogenous lipids. A full characterization of these probes is therefore required for proper interpretation of fluorescence microscopy data in complex membrane systems. Model membrane studies provide essential clues that guide us to design and interpret our experiments, help us to avoid pitfalls and resolve artefacts in complex cellular environments. In the present issue of the Biochemical Journal, Juhasz, Davis and Sharom demonstrate the importance of testing lipid probes systematically in heterogeneous model membranes of specific composition and well-defined thermodynamic properties. The phase-partitioning behaviour of fluorescent probes, alone and/or in combination, cannot simply be assumed, but has to be fully characterized.