CAR: A virus receptor within the tight junction
Introduction
Viruses initiate infection by attaching to receptors on the surface of a susceptible cell. Expression of specific receptors is often an important determinant of a cell's susceptibility to infection and of virus tropism for particular tissues. The coxsackievirus and adenovirus receptor (CAR) was first identified as a cellular protein involved in attachment and infection by group B coxsackieviruses (CVB) and later found to be an adenovirus (Ad) receptor as well [1], [2], [3], [4]. Because of widespread interest in adenoviruses as vectors for therapeutic gene delivery, considerable attention has been paid to CAR's role in virus tropism, and to the structural features important for virus attachment. However, CAR's natural biological functions remain uncertain.
CAR belongs to a growing subfamily of immunoglobulin-like surface molecules, many of which have been localized to sites of cell–cell contact and appear to function in cell adhesion or intercellular recognition. CAR mediates homotypic cell–cell interactions and functions as a transmembrane component of the epithelial cell tight junction [5]. This review will discuss CAR's function as a virus receptor and as a tight junction protein, and describe evidence regarding its broad biological functions.
Section snippets
Protein structure
CAR cDNA encodes a 36O- amino acid protein; cleavage of a 19-residue signal peptide results in a mature protein of 346 amino acids. CAR's predicted molecular weight is approximately 38,000, but it migrates at 46,000 on SDS polyacrylamide gels, most likely due to glycosylation. CAR contains a single membrane-spanning domain that separates an extracellular domain of 216 residues from a 107-residue intracellular domain (Fig. 1A). The cytoplasmic domain contains a site for palmitylation [6],
Evolutionary conservation
CAR has been identified in a variety of mammalian species [3], [24], [30], [31], as well as in non-mammalian vertebrates such as the fish and frog [32], but we have found no obvious CAR homologue in Drosophila or nematodes. Zebrafish and human CAR are 44% identical overall. More extensive conservation within the cytoplasmic domain (66% identity) may suggest that the this portion of the molecule participates in intracellular interactions that are also conserved.
CAR belongs to a family of
Function and association with TJ components
Tight junctions between epithelial cells regulate the flow of ions and macromolecules across the intact epithelium, and serve to divide the apical and basolateral membrane compartments. A variety of evidence indicates that CAR is a component of the tight junction [5]. In polarized epithelial cell lines, and in primary human airway epithelial cells, CAR can be seen–both by confocal microscopy and thin-section electron microscopy–at the apical pole of the lateral membrane, where it colocalizes
CAR tissue distribution
Initial RNA blot analysis suggested that in adults, human CAR is most highly expressed in the heart, brain, and pancreas, with significant levels in the testis and prostate [3], [24]. This RNA expression pattern is consistent with the tropism of coxsackievirus B3, which infects by way of the GI tract, and which causes myocarditis, meningoencephalitis, and pancreatitis. CAR-binding adenoviruses primarily infect the respiratory tract, although these adenoviruses, like CVB, are a major cause of
Development
CAR expression levels change dramatically during development. In the mouse embryo, CAR expression is prominent in the brain, but expression levels drop significantly within the first month of life [11], [94], [95]. High levels of CAR expression in the newborn brain may account for the unique susceptibility of newborn, but not adult, mice to encephalitis caused by CVB [96], [97]. Expression on fetal and regenerating myocytes may account for the susceptibility of these cells, but not adult
Concluding remarks
Since it was isolated in 1997, CAR has been of interest to virologists, gene therapists, and cell biologists. Although it was first identified as a receptor permitting virus attachment, it is now clear that CAR serves an important function in cell–cell interaction. Increasing evidence suggests that CAR functions in the regulation of cell proliferation, and our own recent data indicate that it is essential for normal embryonic development. The mechanisms by which CAR-mediated signals lead to
Acknowledgments
We thank Susan Pichla for help in preparing figures. Our work is supported by grants from the NIH and the American Heart Association.
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