Apical plasma membrane

There is still some discordance about markers for ATI cells [17, 18]. Varieties of glycocalyx appear to be expressed in ATI versus ATII cells, which can be distinguished by the binding of specific lectins. Lycopersicon esculen-tum lectin, for example, appears to adhere specifically to the apical plasma membrane of ATI cells [19, 20], Other lectin markers that have been reported in AEC typing are Ricinus communis agglutinin, Erythrina cristagalli lectin, soybean lectin and Bauhinia purpurea agglutinin (all for the ATI phenotype),... 

An important observation is that certain viruses preferentially bud at different poles of their host cells. In MDCK-cell monolayers, VSV buds exclusively from the basal, or lateral, plasma membranes, and contains sialylated glycoproteins, whereas influenza virus buds exclusively from the apical plasma-membrane, and lacks neuraminic acid. The question arises as to whether glycosylation of viral glycoproteins is needed in order to determine the site of budding. An electron-microscope study revealed that polarity in the maturation sites of these viruses was maintained under conditions of inhibition of glycosyla-... 

The MFGM originates from regions of apical plasma membrane, and also from endoplasmic reticulum (ER) and perhaps other intracellular compartments. That portion of the MFGM derived from apical plasma... 

Figure 10.3 Secretion of lipid globules from bovine mammary epithelial cells. At the time of fixation, three lipid globules (arrows) were in the process of budding from this cell. These globules are partially extruded into the alveolar lumen (L), and the extruded portions are enveloped by a specialized region of apical plasma membrane. Bar = 0.5 /im magnification x 39,000.

The mechanism whereby fat droplets are directed to the apical cytoplasm and become coated with membrane has been the subject of much speculation. There is some evidence that microtubules and microfilaments, elements of the cytoskeleton, may be involved. During lactation a majority of the microtubules in the apical cytoplasm are oriented parallel to the lateral cell surfaces (Nickerson and Keenan 1979 Nickerson et al. 1982). These may act as guides for the directional transfer of lipid droplets to the apical plasma membrane. How-... 

In spite of the fact that the driving forces remain speculative, there is now little doubt that, at secretion, lipid droplets are enveloped in apical plasma membrane, with perhaps some contribution from secretory vesicle membrane. Many questions remain, however, regarding the nature and origin of the inner coat material which lies between the triacylglycerol core and the outer bilayer membrane. To what extent is this material derived from the amorphous surface material seen on lipovesicles within the cell (Dylewski et al. 1984 Deeney et al. 1985 Keenan and Dylewski 1985) and the electron-dense coat on the cytoplasmic face of the apical plasma membrane (Franke et al. 1981) Also to be considered is the clathrin-like coat observed on the outer surface of secretory vesicles (Franke et al. 1976 Mather and Keenan 1983), which may contribute a substantial quantity of material to the lipid droplets (Franke and Keenan 1979), if secretory vesicles do indeed contribute to the formation of the milk lipid globule membrane. 

The extent to which membranes isolated from washed lipid globules originate from the apical plasma membrane, with perhaps some contribution from mature secretory vesicle membrane, can be an important consideration. Any intracellular membranes, contained in cytoplasmic crescents, will be present in preparations obtained by any of the meth-... 

Heid, H. W., Winter, S., Bruder, G., Keenan, T. W. and Jarasch, E.-D. 1983. Butyrophi-lin, an apical plasma membrane-associated glycoprotein characteristic of lactating mammary glands of diverse species. Biochim. Biophys. Acta 728, 228-238. 

It increases the water permeability of the kidney collecting duct cells by inducing translocation of aqua-porin proteins from intracellular storage vesicles into the apical plasma membrane.78 Vasopressin binds to... 

Ameen, N. A., Figueroa, Y., and Salas, P. J. (2001a). Anomalous apical plasma membrane phenotype in CK8-deficient mice indicates a novel role for intermediate filaments in the polarization of simple epithelia. J. Cell Sci. 114, 563-575. 

Ishikawa Y, Skowronski M T, Inoue N, Ishida H. 1999. ai-Adrenoceptor-induced trafficking of aquaporin-5 to the apical plasma membrane of rat parotid cells. Biochem Biophys Res Com 265 94-100. 

Fig. 6.2. Model for how FcRn rescues IgG from catabolism by recycling and transcytosis. IgG and many other soluble proteins are present in extracellular fluids. Vascular endothelial cells are active in fluid phase endocytosis of blood proteins. Material taken up by these cells enters the endosomes where FcRn is found as an integral membrane protein. The IgG then binds FcRn in this acidic environment. This binding results in transport of the IgG to the apical plasma membrane for recycling into the circulation, or to the basolateral membrane for transcytosis into the extracellular space. Exposure to a neutral pFI in both locations then results in the release of IgG. The remaining soluble proteins are channeled to the lysosomal degradation pathway.

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