Microvillus plasma membrane

Alkaline phosphatase is one of the principal glycoproteins present in the microvillus plasma membrane of human placenta sixteen protein and ten glycoprotein subunits of the enzyme were detected. Purified brush-border peptidases (mol. wt. 2.8 x 10 ) from rat intestinal mucosa have been shown to... 

EPEC causes a degeneration of the microvillus brush border, with cupping and pedestal formation of the plasma membrane at the sites of bacterial attachment and reorganization of cytoskeletal proteins [43, 44], Invasion has been observed in some clinical specimens, but the mechanism of how this bacteria produces diarrhea is not fully understood. Some possibilities include an increase in permeability and loss in microvilli leading to malabsorption. 

Villin is an example of a bundling protein. Villin is found in the microvilli of, for example, intestinal brush border cells (Fig. 5-30). The microvilli greatly increase the surface area of the cells, which is essential for effective absorption to take place. Each microvillus extends about 2 p.m into the lumen of the gut and is supported by 20 or so actin filaments tightly bundled by villin (and other proteins) at regular intervals. In a feature common to many actin-based networks, all the filaments in the bundle are oriented with their barbed ends in the same direction, in this case toward the tip of the microvillus where they terminate. Cross-linking of the actin filaments to the plasma membrane occurs via a second protein from the myosin-1 family (a relative of the well-known contractile protein myosin-II). This protein binds its head domain to the sides of the filaments and embeds its tail domain into the membrane. 

Figure 11.12. Electron Micrograph of a Microvillus. Lactase and other enzymes that hydrolyze carbohydrates are present on microvilli that project from the outer face of the plasma membrane of intestinal epithelial cells. [From M. S. Mooseker and L. G. Tilney, J. Cell. Biol. 67(1975) 725.]...

The plasma membrane of the brush border microvilli is characterized by certain distinctive structural features, which may be related to the specialized functional properties that distinguish it from plasma membranes of other cells. The width of the microvillus membrane (measured by electron microscopy) is 10-11 nm, whereas the average eukaryotic plasma membrane is only 7-9 nm. This is probably due to the biochemical composition of the membrane, which is characterized by a high protein to lipid ratio (1.7 1) and a unique lipid composition. The cholesterol-phospholipid ratio and the molar ratio of glycolipid to phospholipid are both about 1 1, which is consistent with the low values for membrane fluidity determined in microvillus membranes. This should be compared with the corresponding ratios... 

Crane (1966) has theorized that the brush border plasma membrane is the site of a mosaic of the enzymes associated with the microvillus. This was based on experiments of Eichholz and Crane (1965) who recovered a fraction of pure microvillous membranes by density gradient centrifugation of brush border homogenate which possessed the total activities of alkaline phosphatase, maltas and sucrase and various peptidases. Evidence in favor of this idea was also collected by Johnson (1967) who demonstrated the presence of knobs 60 A in diameter on the glycocalyx of the luminal side of the plasma membrane which contained the brush border invertase and maltase (see Fig. 6). These knobs could be removed entirely from the microvilli of hamster intestine by papain digestion the remaining membrane, however, still has the alkaline phosphatase incorporated into it (Eichholz, 1969 Oda and Seki, 1966). 

Figure 4. Electron micrograph of the apical portions of several intestinal absorptive cells in the fasted state. Absorptive cells are highly polar. The plasma membrane bordering on the lumen of the intestine is modified to form numerous microvilli (MV). Beneath the microvillus border is the terminal web (TW) zone of the cytoplasm which contains relatively few cytoplasmic organelles. The smooth endoplasmic reticulum (SER) is most prominent just below the terminal web. The RER extends from the region of the terminal web all the way to the supranuclear Golgi complex. Mitochondria (M) nucleus (N) plasma membrane (PM). X 15,940. Reduced 12% for reproduction. From Friedman and Cardell (1977) with permission of the publisher.

Biotin concentrations are 3- to 17-fold greater in plasma from human fetuses compared to those in their mothers in the second trimester, consistent with active placental transport. The microvillus membrane of the placenta contains a saturable transport system for biotin that is Na dependent and actively accumulates biotin within the placenta, consistent with SMVT. 

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