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Rhodopsin is essential for photoreceptor morphogenesis; photoreceptors lacking rhodopsin degenerate in humans, mice, and Drosophila. Here we report that transgenic expression of a dominant-active Drosophila Rho guanosine triphosphatase, Drac1, rescued photoreceptor morphogenesis in rhodopsin-null mutants; expression of dominant-negative Drac1 resulted in a phenotype similar to that seen in rhodopsin-null mutants. Drac1 was localized in a specialization of the photoreceptor cortical actin cytoskeleton, which was lost in rhodopsin-- null mutants. Thus, rhodopsin appears to organize the actin cytoskeleton through Drac1, contributing a structural support essential for photoreceptor morphogenesis.
Sensory neurons present a challenge for morphogenesis: to harness the generic mechanisms of the cytoskeleton to shape a cell to the needs of its specific sensory protein. For photoreceptors, it is clear that morphogenesis and maintenance of the photosensitive organelle, the rhabdomeres of Drosophila and the outer segments vertebrate rods and cones, depend on their sensory protein, rhodopsin (1-3). Rhabdomeres and outer segments are orderly stacks of photosensitive plasma membrane organized from enormously expanded apical cell surfaces. The forces that constrain this expansion and organize it into a dense stack are incompletely understood, but the cortical actin cytoskeleton and its associated proteins are substantial contributors (4, 5). We suggest that in addition to its sensory role, Drosophila rhodopsin organizes the cortical actin cytoskeleton into an essential morphogenetic constraint (6), the rhabdomere terminal web (RTW). The RTW defines the regular, curving base of the rhabdomere that partitions the rhabdomere from the photoreceptor cytoplasm. In rhodopsin-null mutants, the rhabdomere base fails to organize correctly, and the rhabdomere collapses deep into the photoreceptor cytoplasm in convoluted sheets of apposed membrane (1).
A chimeric protein that decorates F-actin with green fluorescent protein (GFP) (7) revealed the RTW as bundled microfilaments extending from the rhabdomere base deep into the photoreceptor (Fig. 1). Before rhodopsin expression, the RTW of developing photoreceptors showed less microfilament bundling, resembling a house painter's brush (Fig. 1C). At about 90% of pupal development (pd), after the onset of rhodopsin expression at 75% pd, RTW microfilaments elongated commensurate with the increasing microvillar length and gathered into bundles (Fig. 1D).
RTW maturation and rhabdomere morphogenesis fail in photoreceptors lacking rhodopsin. Paralleling the normal initiation of microvillar organization observed in rhodopsin-null mutants, the RTW of mutant photoreceptors appeared normal before the...