Intracellular sorting of the plgR

After synthesis and core glycosylation in the rough endoplasmic reticulum of the epithelial cell, the plgR further matures in the Golgi apparatus; it is then sorted in the trans-Golgi network for delivery to the basolateral plasma membrane (Figure l). Both ligand-complexed and unoccupied plgR is internalized by endocytosis in coated pits and sorted for transcytosis in the basolateral endosomal compartment; binding of pig to the receptor is in fact not mandatory for its transport to the luminal cell surface. Here bound and free SC molecules are released by cleavage from the transmembrane anchor by a leupeptine-sensitive protease, while the remaining segment is degraded. Transmembrane SC has therefore been called a 'sacrificial receptor'.

The complex intracellular routing of the plgR through the epithelial cell has made it a model protein for the characterization of intracellular sorting signals. Several studies by Mostov and coworkers based on mutants of rabbit plgR have characterized the function of its cytoplasmic tail (103 residues in human) as an effector segment for routing through the epithelial cell; it apparently carries all the information needed to provide proper delivery at the different cellular locations, including an autonomous basolateral targeting signal, two tyrosine-based endocytosis motifs, and one major phosphorylation site (Figure 2). Phosphorylation of this serine (Ser664) in rabbit plgR (equivalent to Ser655 in human) stimulates transcytosis, which for plgA-receptor complexes in addition is upregulated by ligand binding, probably through second messenger systems. The high conservation of the a hclical membrane spanning domain, including a kink-inducing proline, further suggests that the plgR is capable of mediating signal transduction.

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