Abstract
Drosophila photoreceptors (R cells) are an extreme instance of sensory membrane amplification via apical microvilli, a widely deployed and deeply conserved operation of polarized epithelial cells. Developmental rotation of R cell apices aligns rhabdomere microvilli across the optical axis and enables enormous membrane expansion in a new, proximal distal dimension. R cell ectoplasm, the specialized cortical cytoplasm abutting the rhabdomere is likewise enormously amplified. Ectoplasm is dominated by the actin-rich terminal web, a conserved operational domain of the ancient vesicle-transport motor, Myosin V. R cells harness Myosin V to move two distinct cargoes, the biosynthetic traffic that builds the rhabdomere during development, and the migration of pigment granules that mediates the adaptive "longitudinal pupil" in adults, using two distinct Rab proteins. Ectoplasm further shapes a distinct cortical endosome compartment, the subrhabdomeral cisterna (SRC), vital to normal cell function. Reticulon, a protein that promotes endomembrane curvature, marks the SRC. R cell visual arrestin 2 (Arr2) is predominantly cytoplasmic in dark-adapted photoreceptors but on illumination it translocates to the rhabdomere, where it quenches ongoing photosignaling by binding to activated metarhodopsin. Arr2 translocation is "powered" by diffusion; a motor is not required to move Arr2 and ectoplasm does not obstruct its rapid diffusion to the rhabdomere.