Arc controls organ architecture through modulation of Crb and MyoII

Precise orchestration of morphogenetic processes is required to generate organs that are optimally situated within the organism, and that are of the right size and shape to fit and maximize functionality. Here, we describe the role of Arc, a large apical membrane-associated PDZ domain-containing protein, that works through the apical determinant Crumb (Crb) to limit MyoII activity during tissue invagination in the forming Drosophila salivary gland (SG). We show that loss of Arc, attenuation of Crb function, as well as increased activation of non-muscle Myosin II (MyoII) leads to the simultaneous internalization of more precursor cells than normal. Consequently, mature SGs are significantly shorter with more cells surrounding the lumen at all positions along the tube. Correspondingly, overexpression of Arc or SG-specific knockdown of MyoII leads to the formation of longer SGs with fewer cells surrounding the lumen. We show that both Arc PDZ domains are required for Arc function and that they have distinct activities. Finally, we show that Arc facilitates Crb plasma membrane (PM) localization and suggests a model wherein PM-associated Crb stabilizes cellular junctions countering the destabilizing effects of apical medial and junctional pools of activated MyoII, thus limiting the number of primordial cells internalizing at any given time.