In situ inhibition of vesicle transport and protein processing in the dominant negative Rab1 mutant of Drosophila.

Rab proteins play an essential role in vesicle transport. In particular, RAB1 is thought to participate in the transport of most membrane and secretory proteins. To investigate the role of RAB1 in developing or functioning cells in situ, we constructed transgenic, dominant-negative Rab1 mutants of Drosophila, and examined the protein transport and cellular and subcellular structures of mutant photoreceptor cells. In the transgenic fly, the expression of mutant RAB1 was induced by Gal4 protein, whose expression was triggered by heat treatment (37 degrees C) of the fly. Within several hours after the heat induction, the lumens of the rough endoplasmic reticulum (rER) became swollen, and Golgi bodies were disassembled into vesicle clusters. Corresponding to these changes in cell structure, rhodopsin transport was blocked between the rER and the Golgi body, as indicated by the accumulation of immature rhodopsin carrying a large high-mannose-type oligosaccharide chain. Long-term expression of mutant RAB1 caused the degradation of photoreceptive microvilli and the accumulation of numerous swollen rERs, whereas no distinct changes were found in the axonal regions. These results indicate that, in Drosophila photoreceptor cells, RAB1 contributes to the maintenance of local cell structure by mediating vesicle transport between the rER and Golgi body.