Читать книгу Introduction to the Human Cell - Danton PhD O'Day - Страница 16

Thus proteins are known to exist in domains. But the membrane isn't just made up of a continuous bilayer in which proteins and protein domains reside. There are discontinuities within it. Lipid domains or “rafts” have been shown to exist which contain different concentrations of certain lipids such as cholesterol and sphingolipids. Sphingolipids are a special class of phospholipids involved in cell communication. These rafts are considered to be sites where other specific molecules group for specific functions.

As indicated in the following figure, caveolae (“little caves”) were first seen in the electron microscope as distinct invaginations (infoldings) of the cell membrane (Figure 2.11). Caveolae are a special type of lipid raft that has small caveolin protein molecules localized on their cytoplasmic side. It is likely that the accumulation of many proteins makes the caveolae lipid rafts detectable in the electron microscope.

Figure 2.11. Small caveolae which contain the protein caveolin were seen in electron micrographs.

Caveolae have been implicated in the uptake of cholesterol by endocytosis and in the accumulation of signal transduction and other components prior to their endocytosis by receptor-mediated endocytosis (detailed in Chapter 14). While caveolae are known to be stable, cholesterol-rich membrane domains containing the structure-specific protein caveolin, their potentially diverse roles in cell function are under analysis.

The membrane components are formed in the Golgi and inserted into the cell membrane but much remains to be learned about their biogenesis. The identification of lipid rafts and caveolae reveals that there is much more to be learned about the structure and function of the cell membrane. Later we’ll discuss how vesicles derived from caveolae can move across epithelial cells in the process of transcytosis, an event that underlies bacterial infections caused by Listeria and other pathogens.