Kuzbanian expression levels dictate Notch signaling outcomes via interplay between cis- and trans-Delta.

Spatial regulation of Notch signaling is crucial for tissue patterning, yet how compartment-specific activation thresholds are set remains unclear. Here, we identify Kuzbanian (Kuz) expression as a key spatially controlled determinant in the Drosophila midgut. Kuz is suppressed in the copper cell region by BMP signaling and induced by EGFR activity in adjacent compartments, directly explaining regional differences in Notch activation. Strikingly, elevated Kuz expression alone is sufficient to cleave Notch and trigger ligand-independent signaling. cis-Delta potently inhibits this non-canonical activation, establishing it as a key safeguard. Furthermore, high Kuz levels enable trans-Delta ligands on neighboring cells to overcome cis-inhibition. These findings support a model in which spatially defined Kuz expression sets a proteolytic threshold that determines the outcome of competition between cis-inhibition and trans-activation. Our findings reposition Kuz/ADAM10 as a crucial spatial regulator of Notch signaling, providing a new framework for understanding signal integration in vivo.