Fully differentiated cells are often highly polarized in vivo; epithelial cells and neurons are two well-known examples. Epithelial cells contain apical and basolateral plasma membrane domains, while neurons contain an axon, dendrites, and a cell body. Polarized vesicle transport is essential for establishing and maintaining these polarized structures. However, the underlying mechanisms are not well elucidated. Highly polarized fly photoreceptors are a good genetics-based model for studying the mechanism of polarized transport. In a single plane of the retina, 3 distinct plasma membrane domains of many photoreceptors can be observed. The first domain is the photoreceptive membrane domain, i.e., the rhabdomere, which is formed at the center of the apical plasma membrane during pupal development. Proteins involved in photo-transduction—such as the photosensitive molecule rhodopsin1 (Rh1) and the Ca2+ permeable channel TRP—as well as a protein essential for rhabdomere architecture, chaoptin (Chp), specifically localize in the rhabdomeres. The second domain is the peripheral apical domain surrounding the rhabdomere, i.e., the stalk membrane, which is where the apical determinant Crb is localized. The third domain is the basolateral membrane, which is separated from the apical membrane by the adherens junctions. Na+K+ATPase localizes on the basolateral membrane, similar to typical polarized epithelial cells.
One of the advantages of the Drosophila melanogaster model is that forward genetics can be used for genome-wide screening. Thus, to identify the genes essential for polarized membrane transport to the rhabdomeres, we performed retinal mosaic screening using the FLP/FRT method with in vivo fluorescent imaging of Arrestin2::GFP, which specifically found to activated Rh1. We found several important key players for Rh1 synthesis and transport toward the rhabdomeres: please look at the published papers.
(Updated 2016/10)