PROJECT SUMMARY Actin-based protrusions endow cells with a vast variety of forms. Cells on the surface of mucosal epithelial tissues, such as the cornea and mouth, often project elongated, wrinkle-like protrusions called microridges, which are arranged on apical surfaces in maze-like patterns. Microridges help these cells retain mucus, thus protecting vulnerable epithelial tissues from infection, abrasion and drying out. Until recently, almost nothing was known about microridge morphogenesis. Since microridge morphologies are distinct from those of better- studied protrusions, studying them could reveal fundamental new principles of protrusion morphogenesis. In recent years, the Sagasti lab established larval zebrafish skin cells as a model for studying microridge morphogenesis. This model enables powerful molecular manipulations and live imaging of morphogenesis. The Sagasti lab’s initial studies revealed several new discoveries, including: 1) The dissection of microridge morphogenesis into four molecularly separable steps, 2) the discovery that myosin contraction in the apical cortex regulates surface tension to permit microridge formation, 3) the identification of Plakin cytolinkers as master regulators of microridge length, 4) the discovery that keratin filaments are integral components of microridges required for their stability and elongation (the first example of intermediate filaments directly contributing to protrusion morphogenesis), and 5) the discovery that cortical myosin minifilaments orchestrate an unusual microridge fission/fusion rearrangement process that facilitates the formation of regular, periodic microridge arrangements on cell surfaces. Collectively, these discoveries have broad implications for how cortical myosin, cytolinkers, and intermediate filaments contribute to protrusion morphogenesis. Building on these achievements and new preliminary data, the Sagasti lab will pursue two future lines of research. First, since cortical contractility creates the biomechanical conditions enabling microridge morphogenesis, they will investigate how cross-linking proteins and cell junctions temporally and spatially tune cortical contractility to promote microridge formation and patterning. Since cortical contractility controls many cellular processes, these experiments will reveal regulatory mechanisms with wide relevance in cell biology. Second, since Plakin cytolinkers are central regulators of microridge initiation and elongation, they will investigate how the structure of Plakins relate to their functional roles in microridge morphogenesis, and use Plakin proteins as handles for identifying new regulatory components of the cytoskeletal scaffold that creates microridge protrusions.