The actomyosin cytoskeleton consists primarily of actin, nonmuscle myosins-2 (NM2s) and associated proteins, and is connected to the apical junctional complex (AJC), which consists of apical tight junctions (TJs) and underlying adherens junctions (AJs). It is anchored to the junctional membrane through complexes of transmembrane, scaffolding and adaptor proteins. The actomyosin cytoskeleton is essential for junction assembly, integrity of cell-cell adhesion, paracellular permeability properties of TJs, and also the mechanical properties of the apical cortex that underlies the plasma membrane. Epithelial cells express two isoforms of cytoplasmic actin, beta-actin and gamma-actin, which display distinct biochemical properties, localizations and functions. Beta-actin is enriched at cellular junctions and along lateral membranes and supports the structure of AJs. Gamma-actin localizes at apical junctions and cortex and maintains the architecture of TJs. In vitro, purified gamma-actin forms a more contractile and stiffer network compared to purified beta-actin. Actin filaments interact with myosins. Three isoforms of NM2s exist, NM2A, NM2B and NM2C and they are present in different amounts within the perijunctional actomyosin ring (PAMR) anchored to AJs. However, the crosstalk between cytoplasmic actin and NM2 isoforms at junctions, and whether cytoplasmic actin isoforms control AJC functions as the mechanic of apical junctions and cortex in vivo are not well understood. In this thesis, I investigate the role of gamma-actin in the organization of the actomyosin cytoskeleton, the molecular composition of the AJC, the barrier function of TJs, the dynamics of cytoplasmic TJ proteins, the mechanics of the TJ and apical cortex. Using knock-out (KO), knock-down (KD) and rescue experiments in different types of cultured epithelial cell lines, I demonstrate that KO of gamma-actin up-regulates the expression and junctional accumulation of beta-actin dependent on NM2A up-regulation. Moreover, KO of gamma-actin increases the TJ-membrane tortuosity through the up-regulation of both beta-actin and NM2A, and decreases the stiffness and tension of the apical cortex. Finally, although gamma-actin does not impact the AJC molecular organization and TJ barrier permeability, I show that it regulates the dynamic exchange of the cytoplasmic TJ proteins ZO-1 and CGN. In summary, in my thesis I uncover a new feedback regulatory mechanism by which gamma-actin up-regulates beta-actin through the up-regulation of NM2A and reveal new functions of gamma-actin in the modulation of apical membranes mechanics and TJ protein dynamics.