Xk

Direct apical 1

sorting

Endocytosis

Trans-Golgi network "

Endocytosis

Direct apical 1

sorting

Transcytosis

Tight junction

Apical protein

Apical plasma membrane

Clathrin coated pit

Basolateral plasma membrane

Recycling . j

Transcytosis

Apical protein

Tight junction

Apical plasma membrane virus is transported from the Golgi complex exclusively to the apical membrane, and the VSV G protein is transported only to the basolateral membrane (Figure 17-26). Furthermore, when the gene encoding HA protein is introduced into uninfected cells by recombinant DNA techniques, all the expressed HA accumulates in the apical membrane, indicating that the sorting signal resides in the HA glycopro-tein itself and not in other viral proteins produced during viral infection.

Among the cellular proteins that undergo similar apical-basolateral sorting in the Golgi are those with a glyco-sylphosphatidylinositol (GPI) membrane anchor. In MDCK cells and most other types of epithelial cells, GPI-anchored proteins are targeted to the apical membrane. In membranes GPI-anchored proteins are clustered into lipid rafts, which are rich in sphingolipids (see Figure 5-10). This finding suggests that lipid rafts are localized to the apical membrane along with proteins that preferentially partition into them in many cells. However, the GPI anchor is not an apical sorting signal in all polarized cells; in thyroid cells, for example, GPI-anchored proteins are targeted to the basolateral membrane. Other than GPI anchors no unique sequences have been identified that are both necessary and sufficient to target proteins to either the apical or basolateral domain. Instead, each membrane protein may contain multiple sorting signals, any one of which can target it to the appropriate plasma-membrane domain. The identification of such complex signals and of the vesicle coat proteins that recognize them is currently being pursued for a number of different proteins that are sorted to specific plasma-membrane domains of polarized epithelial cells.

Another mechanism for sorting apical and basolateral proteins, also illustrated in Figure 17-26, operates in hepa-tocytes. The basolateral membranes of hepatocytes face the blood (as in intestinal epithelial cells), and the apical membranes line the small intercellular channels into which bile is secreted. In hepatocytes, newly made apical and basolateral proteins are first transported in vesicles from the trans-Golgi network to the basolateral region and incorporated into the plasma membrane by exocytosis (i.e., fusion of the vesicle membrane with the plasma membrane). From there, both ba-solateral and apical proteins are endocytosed in the same vesicles, but then their paths diverge. The endocytosed ba-solateral proteins are sorted into transport vesicles that recycle them to the basolateral membrane. In contrast, the apically destined endocytosed proteins are sorted into transport vesicles that move across the cell and fuse with the apical membrane, a process called transcytosis. As discussed in the next section, transcytosis also is used to move extracellular materials from one side of an epithelium to another. Even in epithelial cells, such as MDCK cells, in which apical-basolateral protein sorting occurs in the Golgi, transcytosis may provide a "fail-safe" sorting mechanism. That is, an apical protein sorted incorrectly to the basolateral membrane would be subjected to endocytosis and then correctly delivered to the apical membrane.

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