Shroom2, a myosin-VIIa- and actin-binding protein, directly interacts with ZO-1 at tight junctions

Defects in myosin VIIa lead to developmental anomalies of the auditory and visual sensory cells. We sought proteins interacting with the myosin VIIa tail by using the yeast two-hybrid system. Here, we report on shroom2, a submembranous PDZ domain-containing protein that is associated with the tight junctions in multiple embryonic and adult epithelia. Shroom2 directly interacts with the C-terminal MyTH4-FERM domain of myosin VIIa and with F-actin. In addition, a shroom2 fragment containing the region of interaction with F-actin was able to protect actin filaments from cytochalasin-D-induced disruption in MDCK cells. Transfection experiments in MDCK and LE (L fibroblasts that express E-cadherin) cells led us to conclude that shroom2 is targeted to the cell-cell junctions in the presence of tight junctions only. In Ca²⁺-switch experiments on MDCK cells, ZO-1 (also known as TJP1) preceded GFP-tagged shroom2 at the differentiating tight junctions. ZO-1 directly interacts with the serine- and proline-rich region of shroom2 in vitro. Moreover, the two proteins colocalize in vivo at mature tight junctions, and could be coimmunoprecipitated from brain and cochlear extracts. We suggest that shroom2 and ZO-1 form a tight-junction-associated scaffolding complex, possibly linked to myosin VIIa, that bridges the junctional membrane to the underlying cytoskeleton, thereby contributing to the stabilization of these junctions.