Sorting proteins to their target membranes.

The functional polarity of epithelial cells depends upon the selective insertion of proteins and lipids into distinct plasma membrane domains, and upon the maintenance of these specialized domains once they are established during epithelial development. This polarized distribution of important categories of proteins including membrane transporters, channels, enzymes, cell adhesion molecules and junctional components allows cells to carry out the vectorial transport of fluid, ions and other molecules across the epithelial barrier. Several mechanisms are required to ensure the directed movement of membrane components within the cell, and to control their delivery to the appropriate target membrane. These include specific "targeting" cassettes in the amino acid sequence of the transported proteins (such as PDZ domains and NPXY or YRRF domains), a variety of accessory proteins (including GTP-binding proteins) that associate with carrier vesicles and membrane compartments within the cell, and cytoskeletal elements such as microtubules, microfilaments and the spectrin-ankyrin network. Incorrectly folded proteins are retained and degraded within the cell, and many "chaperones" are involved in ensuring that newly-synthesized proteins assume the correct two- and three-dimensional orientations and oligomerization prior to exiting from the endoplasmic reticulum (ER). Many of the proteins involved in neurotransmitter release (for example, synaptobrevins, syntaxins) have homologs that are found in non-neuronal cells, where they play a key role in vesicle fusion with the plasma membrane. In view of the complexity of these trafficking processes, it is not surprising that a growing number of disease pathologies have been identified that involve defective targeting and trafficking of proteins. These diseases can be grouped under the name "sorting disorders," and they result from abnormal delivery of functionally important proteins to the cell surface. In some cases, the mutated protein is retained and degraded intracellularly, while in others it may not be delivered to the cell surface after the appropriate physiological stimulation.