Spatial deconstruction of the plasma membrane

The plasma membrane of cells is known to be heterogeneous with regards to the spatial distribution of proteins and lipids. The segregation of molecular species in the cell membrane into specific domains permits different biological pathways to function independently. But the nature of these domains and how they are organized is unclear. Here we perform fluorescence microscopy on plasma membrane sheets to show that most proteins occur in "protein-rich domains", while other regions of the cell membrane are relatively protein-free. We show that this organization is at least partially preserved in plasma membrane-derived vesicles that lack cytoskeletal elements, and then demonstrate biochemical separation of the protein-rich and protein-poor domains using density centrifugation. The isolation of these spatial domains allows us to study their composition using analytical methods. Most types of proteins and ordered lipids including cholesterol are enriched in the protein-rich domains, indicating preferential self-assembly between specific proteins and lipids. Significance StatementThe plasma membrane of living cells performs many different functions that enable the transfer of ions, molecules and information signals between a cell and its environment. We show that the plasma membrane is divided into separate compartments that can be isolated and analyzed. Protein-rich compartments exhibit a high density of certain classes of proteins and lipids. Protein-poor compartments are more sparsely populated by other kinds of proteins. The separate compartments fulfill different functions. The cell membrane is thus organized to regionalize separate tasks. The underlying organization appears to arise through spontaneous self-assembly of membrane components.