Abstract
In 1972 the Fluid—Mosaic Membrane Model of membrane structure was proposed based on thermodynamic principals of organization of membrane lipids and proteins and available evidence of asymmetry and lateral mobility within the membrane matrix [S. J. Singer and G. L. Nicolson, Science 175 (1972) 720–731]. After over 40years, this basic model of the cell membrane remains relevant for describing the basic nano-structures of a variety of intracellular and cellular membranes of plant and animal cells and lower forms of life. In the intervening years, however, new information has documented the importance and roles of specialized membrane domains, such as lipid rafts and protein/glycoprotein complexes, in describing the macrostructure, dynamics and functions of cellular membranes as well as the roles of membrane-associated cytoskeletal fences and extracellular matrix structures in limiting the lateral diffusion and range of motion of membrane components. These newer data build on the foundation of the original model and add new layers of complexity and hierarchy, but the concepts described in the original model are still applicable today. In updated versions of the model more emphasis has been placed on the mosaic nature of the macrostructure of cellular membranes where many protein and lipid components are limited in their rotational and lateral motilities in the membrane plane, especially in their natural states where lipid–lipid, protein–protein and lipid–protein interactions as well as cell–matrix, cell–cell and intracellular membrane-associated protein and cytoskeletal interactions are important in restraining the lateral motility and range of motion of particular membrane components. The formation of specialized membrane domains and the presence of tightly packed integral membrane protein complexes due to membrane-associated fences, fenceposts and other structures are considered very important in describing membrane dynamics and architecture. These structures along with membrane-associated cytoskeletal and extracellular structures maintain the long-range, non-random mosaic macro-organization of membranes, while smaller membrane nano- and submicro-sized domains, such as lipid rafts and protein complexes, are important in maintaining specialized membrane structures that are in cooperative dynamic flux in a crowded membrane plane. This Article is Part of a Special Issue Entitled: Membrane Structure and Function: Relevance in the Cell's Physiology, Pathology and Therapy.
Highlights
When the Fluid—Mosaic Membrane Model (F-MMM) of biological membrane structure was first introduced in 1972, it was envisioned as a basic framework model for cell membranes that could explain existing data on membrane proteins and lipid structures and their dynamics and help plan and predict future experimental outcomes [1]
Lipids that support positive spontaneous curvature can reverse the effects of lipids that support negative spontaneous curvature to maintain membrane form, and this difference may be important in membrane fusion, fission and other membrane–membrane interactions. Another thermodynamic consideration of the F-MMM was that since the free energy required to flip membrane lipids and proteins across the hydrophobic membrane interior would be substantial, cell membrane flip-flop that could result in symmetric structures should be exceptionally low [1,49]
The compositional differences between the inner and outer leaflets of the cell membrane lipid bilayer suggest that the outer leaflet is curvature neutral, whereas the inner leaflet may have a preference for negative curvature, or as Zimmerberg and Gawrich state in their review, at the inner surface the polar interface has a smaller lateral area than the hydrocarbon chain region and drives a net curvature to minimize total curvature energy of the bilayer [73]
Summary
When the Fluid—Mosaic Membrane Model (F-MMM) of biological membrane structure was first introduced in 1972, it was envisioned as a basic framework model for cell membranes that could explain existing data on membrane proteins and lipid structures and their dynamics and help plan and predict future experimental outcomes [1]. I have re-termed the model as the ‘Fluid—Mosaic Membrane Model’ to highlight the important role of mosaic, aggregate and domain structures in membranes and the restraints on lateral mobility of many if not most membrane protein components This designation was done not to revise history or justify claims that were never part of the original model; it was done to make the model more consistent with newer information that was not available in 1972. Viruses, cell junctions, adhesion sites, lipid rafts, mitochondrial inner membranes and other compact membranous structures possess limited lateral macro-mobility of specific membrane components while still exhibiting the basic microstructure of the F-MMM This will be considered in later sections of this review. Due to the vast literature on various cellular membranes that could not be carefully considered in a single review it has been necessary here to concentrate on cell or plasma membrane structure and function
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