Afadin controls cadherin cluster stability using clathrin-independent mechanism

Afadin is an actin-binding protein that interacts with the intracellular region of the transmembrane proteins, nectins. In collaboration with other transmembrane proteins, cadherins, nectins form adherens junctions, a major type of cell-cell adhesive structures in the multicellular organisms. To elucidate the afadin function, we studied adherens junction defects induced by afadin depletion in epithelial A431 cells. We have found that the cells lacking afadin exhibit no abnormalities in morphology or in general dynamics of adherens junctions in the confluent cell cultures. The only observed difference is a slight increase in the rate of cadherin turnover in these junctions. However, afadin depletion strongly affects the assembly of new adherens junctions immediately after two cells touch one another: initiation of new junctions is significantly delayed, the growth of the nascent junctions stagnates, and their lifetime shortens. As a result, the afadin-depleted cells need much more time to establish the mature junctional structures. This defect is not caused by the clathrin-dependent endocytosis of cadherin clusters that was monitored using live-cell imaging of A431 cells co-expressing GFP-tagged E-cadherin and mCherry-tagged clathrin light chain. Taken together our data show that afadin reinforces adherens junctions and that this process is crucial for the fast formation of adherens junctions at the sites of new cell-cell contacts.