Unit of Cellular Morphogenesis

Drosophila photoreceptors are primary sensory cells differentiated from typical polarized epithelial cells. Morphogenesis of the photosensitive membrane , the rhabdomere, is driven by a late-pupal surge of secretory traffic that greatly expands the apical plasma membrane in a column of closely packed, rhodopsin-rich photosensitive microvilli. Since photoreceptor morphogenesis places great demands on cytoskeletal organization and vesicle traffic, developing photoreceptors are sensitive to the impairment of these systems. Together with the advanced molecular genetics, the study of Drosophila photoreceptors can be a useful system to identify and investigate these fundamental mechanisms of molecular cell biology.

The mechanism of Rhodopsin transport

Figure 1. Rh1-bearing post-Golgi vesicles contain Rab11/dRip11/MyoV complex. MyoV powers vesicles through RTW cytoplasm.

A distinctive feature of the system that we have developed is access to dynamics of protein transport. A Rhodopsin consists of a protein component, opsin, and a chromophore, 11-cis retinal. Without chromophores, opsin is synthesized on, but does not exit from the ER. Photoisomerization of all-trans-retinal to 11-cis retinal by blue light illumination induces synchronous release of Rhodopsin from the ER into the secretory pathway. Using this technique, we have shown that Rab1 and Rab11 are required for Rhodopsin transport from ER to Golgi and Golgi to the rhabdomere, respectively (Satoh et al., 1997, Satoh et al., 2005). A dense microfilament web, the rhabdomere terminal web (RTW) extends into photoreceptor cytoplasm from the rhabdomere base; RTW microfilaments are oriented with their plus ends toward the rhabdomere base and blocking incursion of vesicles and proteins. We have shown that Rab11 forms a trio-protein complex with dRip11 and MyoV, and that MyoV powers Rhodopsin-bearing post-Golgi vesicles into the RTW; thus the vesicles are transported to the photoreceptive rhabdomere membrane (Li et al., 2007). Currently, we are working to identify additional molecules involved in this process.

The mechanisms of pigment granule motility.

Pigment granule migration mediates an automatic gain control in fly photoreceptors; in response to light, small pigment granules migrate to the base of the photosensitive membrane organelle, the rhabdomere, where they attenuate waveguide propagation along the rhabdomere and reduce photoreceptor sensitivity approximately two orders of magnitude.
From the electron microscopic observation of pigmant granule position on the dark–adapted photoreceptors fixed with infrared illumination and image intensifier eyepieces, we found piment granules line up at the behind of the RTW. We have shown that pigment granule migration is mediated by myosin V, calmodurin and a Rab family protein, Lightoid (Satoh et al., 2008). Currently, we are working to identify additional molecules involved in this process.

Lab members