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Am J Physiol Cell Physiol (September 13, 2007). doi:10.1152/ajpcell.00309.2007
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Submitted on July 19, 2007
Accepted on September 4, 2007

Tight junction targeting and intracellular trafficking of occludin in polarized epithelial cells

Veedamali S Subramanian1, Jonathan S Marchant2, Dongmei Ye3, Thomas Y Ma4, and Hamid M. Said5*

1 Medicine, UCI/VA Medical Program, LOng Beach, California, United States
2 Pharmacology, University of Minnesota Medical School, Minneapolis, Minnesota, United States
3 Internal Medicine, University of New Mexico, New Mexico, New Mexico, United States
4 New Mexico, New Mexico, United States; Internal Medicine, University of New Mexico, New Mexico, New Mexico, United States
5 Medical Research, UCI/VA Medical Center, Long Beach, California, United States; Medicine & Physiology/ Biophysics, Univerisity of California, Medical Scinces I, C-354, Irvine, California, 92697, United States

* To whom correspondence should be addressed. E-mail: hmsaid{at}uci.edu.

Occludin, a transmembrane spanning protein, is an integral component of the tight junctional (TJ) complexes that regulate epithelial integrity and paracellular barrier function. However, the molecular determinants that dictate occludin targeting and delivery to the TJ remain unclear. Here, using live cell imaging of YFP-labeled occludin fragments, we resolved the intracellular trafficking of occludin-fusion proteins in polarized MDCK and Caco-2 cells to delineate regions within the occludin polypeptide important for occludin targeting to the TJs. Live cell confocal imaging revealed that complete, or partial truncation of the COOH-terminal tail of the occludin polypeptide did not prevent occludin targeting to the TJ in epithelial cell lines. Progressive truncations into the COOH-terminal tail decreased the efficiency of occludin expression, until regions proximal to the fourth transmembrane (TM) domain were removed, when efficiency of expression increased. However, further deletions into TM4 abolished TJ targeting resulting in constructs that were retained intracellularly within the endoplasmic reticulum. The full length occludin polypeptide trafficked to the cell surface within a heterogenous population of intracellular vesicles that delivered occludin to the plasma membrane in a microtubule- and temperature-dependent manner. In contrast, steady state localization of occludin at the cell surface was dependent on intact microfilaments but not microtubules.




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