Proximity labeling reveals interactions necessary to maintain the distinct apical domains of Drosophila photoreceptors.

Specialized membrane and cortical protein regions are common features of cells and are utilized to isolate differential cellular functions. In Drosophila photoreceptors, the apical membrane domain is defined by two distinct morphological membranes: the rhabdomere microvilli and the stalk membrane. To define the apical cortical protein complexes, we performed proximity labeling screens utilizing the rhabdomeric-specific protein PIP82 as bait. We found that the PIP82 interactome is enriched for actin-binding and cytoskeleton proteins, as well as proteins for cellular trafficking. Analysis of one target, Bifocal, with PIP82 revealed two independent pathways for localization to the rhabdomeric membrane and an additional mechanism of crosstalk between the protein complexes of the rhabdomeric and stalk membranes. The loss of Bifocal, and enhancement in the Bifocal/PIP82 double mutant, results in the additional distribution of Crumbs, an apical stalk membrane protein, to the lateral basal photoreceptor membrane. This phenotype is recapitulated by the knockdown of the catalytic subunit of Protein Phosphatase 1, a known interactor of Bifocal. Taken together, these results expand our understanding of the molecular mechanisms underlying the generation of the two distinct photoreceptor apical domains.