LI-cadherin cis-dimerizes in the plasma membrane Ca(2+) independently and forms highly dynamic trans-contacts

LI-cadherin belongs to the family of 7D-cadherins that is characterized by a low sequence similarity to classical cadherins, seven extracellular cadherin repeats (ECs), and a short cytoplasmic domain. Nevertheless, LI-cadherins mediates Ca(2+)-dependent cell-cell adhesion and induces an epitheloid cellular phenotype in non-polarized CHO cells. Whereas several studies suggest that classical cadherins cis-dimerize in a Ca(2+)-dependent manner and interact in trans by strand-swapping tryptophan 2 of EC1, little is known about the molecular interactions of LI-cadherin, which lacks tryptophan 2. We thus expressed fluorescent LI-cadherin fusion proteins in HEK293 and CHO cells, analyzed their cell-cell adhesive properties and studied their cellular distribution, cis-interaction, and lateral diffusion in the presence and absence of Ca(2+). LI-cadherin highly concentrates in cell contact areas but rapidly leaves those sites upon Ca(2+) depletion and redistributes evenly on the cell surface, indicating that it is only kept in the contact areas by trans-interactions. Fluorescence resonance energy transfer analysis of LI-cadherin-CFP and -YFP revealed that LI-cadherin forms cis-dimers that resist Ca(2+) depletion. As determined by fluorescence redistribution after photobleaching, LI-cadherin freely diffuses in the plasma membrane as a cis-dimer (D = 0.42 ± 0.03 μm(2)/s). When trapped by trans-binding in cell contact areas, its diffusion coefficient decreases only threefold to D = 0.12 ± 0.01 μm(2)/s, revealing that, in contrast to classical and desmosomal cadherins, trans-contacts formed by LI-cadherin are highly dynamic.