Transgenic expression of ceramidase suppresses retinal degeneration in Drosophila arrestin and phospholipase C mutants. Here, we show that expression of ceramidase facilitates the dissolution of incompletely formed and inappropriately located elements of rhabdomeric membranes in ninaE I17 mutants lacking the G protein receptor Rh1 in R1-R6 photoreceptor cells. Ceramidase expression facilitates the endocytic turnover of Rh1. Although ceramidase expression aids the removal of internalized rhodopsin, it does not affect the turnover of Rh1 in photoreceptors maintained in dark, where Rh1 is not activated and thus has a slower turnover and a long half-life. Therefore, the phenotypic consequence of ceramidase expression in photoreceptors is caused by facilitation of endocytosis. This study provides mechanistic insight into the sphingolipid biosynthetic pathway-mediated modulation of endocytosis and suppression of retinal degeneration.S phingolipids are integral components of eukaryotic cell membranes. They are essential for survival of yeast, Drosophila, and mammals. Sphingolipids have a hydrophobic backbone moiety, ceramide, linked to a variable polar head group, e.g., phosphorylcholine in sphingomyelin and a saccharide in glycosphingolipids. Ceramide itself consists of a long chain base linked to a fatty acid. Ceramide, sphingosine, and sphingosine-1-P are lipid second messengers modulating a variety of cellular functions (1-3). Ceramide is at the branch point of the sphingolipid biosynthetic pathway and serves as a substrate for several enzymes, e.g., sphingomyelin synthase, ceramidase, ceramide kinase, and glucosylceramide synthase. In Drosophila, transgenic expression of a neutral ceramidase is accompanied by a decrease in the levels of ceramide (4). We have shown that modulation of enzymes of the sphingolipid biosynthetic pathway suppressed retinal degeneration in certain phototransduction mutants (4). Transgenic expression of ceramidase or a loss of one copy of the rate-limiting enzyme in the sphingolipid biosynthetic pathway, namely, Serine Palmitoyl CoA transferase, suppressed retinal degeneration in arrestin and phospholipase C (norpA) mutants. These backgrounds also suppressed degeneration in a dynamin mutant, suggesting that ceramidase was modulating the endocytic pathway. To gain further insight into the mechanism of ceramidase action, we have investigated the effects of ceramidase expression in the rhodopsin null mutant, ninaE I17 . Rhodopsin, the G protein-coupled receptor, transduces light signal in Drosophila photoreceptors R1-R6 (5, 6). Rhodopsin is also involved in the formation of a rhabdomere terminal web (RTW), an actin-based cytoskeletal scaffold implicated in rhabdomere biogenesis (7,8). Rhabdomeres are densely organized, specialized areas of plasma membrane that house the signal transduction machinery of photoreceptors. In ninaE I17 mutants, the RTW is not organized during morphogenesis and thus photoreceptors have improperly formed rhabdomeres (8). In addition, in 3-dayold flies, rhabdomeric e...