Figure 1.
Panel A shows a top view of epithelial tissue organisation: A grid of hexagonal cells shows three types of vertices where cells meet: a four-cell vertex (transient junction), a six-cell vertex (transient rosette), and a tricellular vertex (stable). A bicellular junction between two cells is also marked, linking to Panel B. Panel B shows a bicellular junction: A cross-section of two adjacent cells, from apical to basal, sitting on the basement membrane or ECM. Five junction types anchor the cells together, listed apical to basal: tight junction, adherens junction using actin, desmosome using keratin, gap junction, and hemidesmosome. Panel C shows resolution of a four-cell vertex into stable tricellular junctions: Five stages show how an unstable four-cell vertex resolves into two stable tricellular junctions, tracking Afadin, PAK4, and Myosin 2 levels at each step: 1. Unresolved: Afadin present, Myosin 2 elevated. 2. PAK4 recruited: Afadin recruits PAK4, Myosin 2 active. 3. Vertex resolution initiated: PAK4 and Myosin 2 active. 4. Myosin 2 restrained: GEF-H1-Rho-ROCK-MYPT1 turns down Myosin 2. Vertex resolved: Stable vertices form, PAK4 undocks from the junction.
PAK4 resolves transient vertices. (A) Schematic of the top view of an epithelial tissue showing bicellular junctions and different vertex configurations. Bicellular junctions, stable tricellular vertices, and transient higher order vertices are indicated. Green cells participate in transient vertices, whereas blue cells form stable tricellular vertices. (B) Schematic of a vertebrate bicellular junction showing the organization of the tight junction and adherens junction in the apical region. (C) Sequential steps during the resolution of a transient four-way vertex into a stable tricellular vertex. The schematic illustrates the proposed roles of Afadin, PAK4, and Myosin II during vertex remodeling.