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Zonula adherens junctions (zAJ) are spatially proximal to tight junctions (TJ), in a superstructure known as the apical junctional complex (AJC). A key component of the AJC is a circumferential ring of filamentous (F)-actin, but how actomyosin contractility drives AJC structure and epithelial barrier function is incompletely understood. Here, we show that a central mechanosensitive component of zAJ, α-catenin (α-cat), undergoes force-dependent phosphorylation in an unstructured linker region. This modification in turn primes the α-cat mechanosensitive Middle-region for effector-binding. We credential Afadin, a multi-domain TJ/AJ scaffold protein, as mechano-chemical binding partner of α-cat, identifying residues in α-cat required for this interaction. α-cat phosphorylation and Afadin-binding are required for their co-enrichment at zAJ and epithelial barrier function. A mouse model that prevents α-cat phosphorylation is particularly detrimental to post-natal brain development. These data support a stepwise model where α-cat integrates mechanical and chemical signals to progressively promote zAJ enrichment, effector recruitment and epithelial barrier function.