The plasma membrane is the theater of all kinds of material and information exchange between a cell and its environment - that is, the "outer world". The plasma membrane is basically a lipid bilayer that accommodates a significant number of proteins with diverse structures and tasks necessary for the proper function of cells. Concepts about the architecture of the plasma membrane have changed dramatically through the past decades. The "rigid membrane" concept was replaced by the Singer-Nicolson (S-N) fluid mosaic membrane model in 1972, which postulated the random distribution and free lateral mobility of proteins in the unstructured lipid environment . Construction of the S-N model was greatly advanced by the classical experiment of Frye and Edidin, which demonstrated intermixing of distinct molecular species in the plasma membrane of heterokar-yons of human and murine lymphocytes . Although the S-N model was able to explain many phenomena taking place in the cell membrane, it had a limited validity. First of all, it was mostly applicable to the quasi-symmetric circulating blood cells. It has been well known for a long time that cells built into solid tissues are frequently polarized as required by their physical environment and biological function, a most extreme example being the organization of brush border cells, into two discrete regions - the apical and the basolateral - with distinct functions and thus different lipid and protein composition. The picture of "freely moving proteins in the uniform lipid sea" was challenged by experimental observations suggesting the locally restricted mobility of proteins in the lipid bilayer [3,4]. Data indicating the existence of hierarchically built protein complexes even in nonpolarized cells also contradicted the S-N model [5,6]. As for the concept of "uniform lipid sea", an important step forward was the discovery of sphingolipid- and cholesterol-enriched lipid domains - that is, lipid rafts - constituting a proof that phase-separation of lipids observed in model membranes also occurs in the more complex plasma membrane of living cells [7,8]. All these observations led to the "membrane microdomain" concept, namely the compartmentalization/organiza-tion of membrane proteins and lipids into nonrandom, well-defined, yet dynamic structures, which exist at different time and size scales.
142 | 7 Role of Lipid Microdomains in the Formation of Supramolecular Protein Complexes 7.1.1
Was this article helpful?
This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.