Lipid Rafts: Controversies Resolved, Mysteries Remain

Abstract

The lipid raft hypothesis postulates that lipid-lipid interactions can laterally organize biological membranes into domains of distinct structures, compositions, and functions. This proposal has in equal measure exhilarated and frustrated membrane research for decades. While the physicochemical principles underlying lipid-driven domains has been explored and is well understood, the existence and relevance of such domains in cells remains elusive, despite decades of research. Here, we review the conceptual underpinnings of the raft hypothesis and critically discuss the supporting and contradicting evidence in cells, focusing on why controversies about the composition, properties, and even the very existence of lipid rafts remain unresolved. Finally, we highlight several recent breakthroughs that may resolve existing controversies and suggest general approaches for moving beyond questions of the existence of rafts and towards understanding their physiological significance.

Figure 1 -. Differential lipid interactions drive lateral organization.

(A) Saturated lipids and cholesterol interact more favorably with each other than with unsaturated lipids. (B) Collectively, these differential interactions oppose the entropic tendency towards random mixing. When the various interactions sufficiently outweigh the demixing cost (dG_demixing ~ −kT), stable lateral domains are formed.

Figure 2 -. Cooperative regulation between membrane proteins and lateral domains.

(A) Freely-diffusing proteins can be recruited to membrane domains by post-translational lipid modifications (e.g. palmitoylations) and features such as long and thin transmembrane domains. (B-D) Proteins can also template and regulate membrane domains. (B) Cytoskeletal protein networks can template membrane domains by tethering specific lipids or proteins. (C) Oligomerization of raft-associated proteins can cluster and stabilize raft domains. (D) Immobilized, membrane-bound scaffolds (e.g. post-synaptic density) may recruit specific membrane domains based on the raft affinity of their membrane-anchoring proteins.

Figure 3 -. Accumulating evidence for raft domains in real life.

(A) A variety of cross-validated probes have recently been developed, allowing comparison of single-molecule diffusion behaviors between raft-enriched and raft-excluded probes. Fluorescence lipid analogs that prefer ordered phases in model and bio-derived membranes also tend to be detergent-resistant and show distinct diffusive behaviors, statistically enriching in GPI-anchored protein-rich zones. (B) Clustering of viral Gag by matrix proteins leads to microscopically observable membrane domains that sort host proteins based on their affinity for ordered lipid phases. (C) Super-resolution microscopy in live cells reveals statistical enrichment of raft preferring proteins near activated B-cell receptors. (D) Some transmembrane proteins rely on raft affinity for their subcellular localization. For these, the TMDs are fully sufficient to recapitulate the trafficking fates of the full-length protein, and their raft affinity is essential, as non-raft mutants are mis-sorted and ultimately degraded.