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A Unique Role for Nonmuscle Myosin Heavy Chain IIA in Regulation of Epithelial Apical Junctions 1 1 1 2 1 1 Andrei I. Ivanov *, Moshe Bachar , Brian A. Babbin , Robert S. Adelstein , Asma Nusrat , Charles A. Parkos 1 Epithelial Pathobiology Research Unit, Department of Pathology and Laboratory Medicine, Emory University, Atlanta, Georgia, United States of America, 2 Laboratory of Molecular Cardiology, National Heart Lung and Blood Institute, National Institute of Health, Bethesda, Maryland, United States of America The integrity and function of the epithelial barrier is dependent on the apical junctional complex (AJC) composed of tight and adherens junctions and regulated by the underlying actin filaments. A major F-actin motor, myosin II, was previously implicated in regulation of the AJC, however direct evidence of the involvement of myosin II in AJC dynamics are lacking and the molecular identity of the myosin II motor that regulates formation and disassembly of apical junctions in mammalian epithelia is unknown. We investigated the role of nonmuscle myosin II (NMMII) heavy chain isoforms, A, B, and C in regulation of epithelial AJC dynamics and function. Expression of the three NMMII isoforms was observed in model intestinal epithelial cell lines, where all isoforms accumulated within the perijunctional F-actin belt. siRNA-mediated downregulation of NMMIIA, but not NMMIIB or NMMIIC expression in SK-CO15 colonic epithelial cells resulted in profound changes of cell morphology and cell-cell adhesions. These changes included acquisition of a fibroblast-like cell shape, defective paracellular barrier, and substantial attenuation of the assembly and disassembly of both adherens and tight junctions. Impaired assembly of the AJC observed