Synaptopodin is required for stress fiber and contractomere assembly at the epithelial junction

The apical junction of epithelial cells can generate force to control cell geometry and perform contractile processes while maintaining barrier function and adhesion. Yet, the structural basis for force generation at the apical junction is not fully understood. Here, we describe 2 synaptopodin-dependent actomyosin structures that are spatially, temporally, and structurally distinct. The first structure is formed by retrograde flow of synaptopodin initiated at the apical junction, creating sarcomeric stress fibers that lie parallel to the junction and insert into junctional complexes on the apical plane. Retrograde flow of synaptopodin is also seen at vinculin-decorated basal junctions on the basal plane. Contractions of apical stress fibers is associated with clustering of membrane complexes via side-on synaptopodin linkers whereas contractions of stress fibers inserted at the apical junction via head-on synaptopodin linkers results in junction shortening. Upon junction maturation, apical stress fibers are disassembled. In mature epithelial monolayer, a motorized “contractomere” complex capable of “walking the junction” is formed at junction vertices. Contractomere motility results in changes in junctional length, altering the overall shape of the cell and packing geometry within the monolayer. We propose a model of epithelial homeostasis that utilizes contractomere motility to preserve the permeability barrier during intercellular movement and junctional processes. Summary Statement Synaptopodin retrograde flows initiate the assembly of apical and basal stress fibers from the apical and basal junctions. In mature apical junction, a motorized junctional complex, we termed the contractomere, allows the apical junction to change length and organize cell geometry within a confluent monolayer. ### Competing Interest Statement The authors have declared no competing interest.