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J. Cell Biol.,
Volume 142, Number 1, July 13, 1998 101-115

* Department of Molecular and Cellular Physiology, Stanford University School of Medicine, Stanford, CA 94305-5345; Tight junctions (TJ) govern ion and solute
diffusion through the paracellular space (gate function), and restrict mixing of membrane proteins and lipids between membrane domains (fence function) of polarized epithelial cells. We examined roles of the RhoA
and Rac1 GTPases in regulating TJ structure and function in MDCK cells using the tetracycline repressible
transactivator to regulate RhoAV14, RhoAN19,
Rac1V12, and Rac1N17 expression. Both constitutively active and dominant negative RhoA or Rac1 perturbed
TJ gate function (transepithelial electrical resistance,
tracer diffusion) in a dose-dependent and reversible
manner. Freeze-fracture EM and immunofluoresence microscopy revealed abnormal TJ strand morphology
and protein (occludin, ZO-1) localization in RhoAV14
and Rac1V12 cells. However, TJ strand morphology
and protein localization appeared normal in RhoAN19
and Rac1N17 cells. All mutant GTPases disrupted the fence function of the TJ (interdomain diffusion of a fluorescent lipid), but targeting and organization of a
membrane protein in the apical membrane were unaffected. Expression levels and protein complexes of occludin and ZO-1 appeared normal in all mutant cells, although ZO-1 was more readily solubilized from
RhoAV14-expressing cells with Triton X-100. These results show that RhoA and Rac1 regulate gate and fence
functions of the TJ, and play a role in the spatial organization of TJ proteins at the apex of the lateral membrane.
Department of Pathology, Massachusetts General Hospital, Boston, MA 02114
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