Rop and Ras2, members of the Sec1 and Ras families, are localized in the outer membranes of labyrinthine channels and vesicles of Drosophila nephrocyte, the Garland cell.

The product of the ras opposite (rop) gene is an essential component of secretion processes in Drosophila. The rop gene product is homologous to the Caenorhabditis elegans UNC-18 and the rat munc-18/n-Sec1/rbSec1 proteins, implicated in the final steps of neurotransmitter exocytosis in nerve terminals, and the bovine mSec1 protein implicated in the secretion of catecholamines in chromaffin cells. The mammalian brain protein has been shown to exert its activity in the presynaptic membrane through transient interaction with syntaxin, an integral component of this membrane. rop is highly expressed in the Drosophila nervous system, where it acts as both a positive and negative modulator of neurotransmitter release. It is also expressed in specialized tissues in which intensive exocytic/endocytic cycles take place, including the garland cells, a small group of nephrocytes which take up waste materials from the hemolymph by endocytosis. rop is regulated by a bidirectional promoter shared with Ras2, a member of the R-ras/TC21 branch of the ras supergene family. Ras2 is also highly expressed in the garland cells. These cells are characterized by their labyrinthine channels, long invaginations extending from the cell membrane, and a rich population of a variety of vesicles. In this study, we analyzed the ultrastructural localization of the Rop and Ras2 proteins in the garland cell. Rop was detected in the outer membranes of the labyrinthine channels, and in the outer membranes of many vesicles located nearby the labyrinthine channels, but not in vesicles located in inner parts of the cell. Using glutathione-S-transferase-syntaxin fusion, we show that Rop is firmly bound to syntaxin.(ABSTRACT TRUNCATED AT 250 WORDS)